Each square represents $\displaystyle 3$ students.  Ms. Miller's bar has $\displaystyle 3$ squares in it, which means she has $\displaystyle 9$ students with pets ($\displaystyle 3\times3=9$). Ms. Hen's bar has $\displaystyle 2$ squares in it, which means she has $\displaystyle 6$ students with pets ($\displaystyle 3\times2=6$).

To find the difference we subtract. 

$\displaystyle 9-6=3$

Each square represents $\displaystyle 3$ students. Mr. Ray's bar has $\displaystyle 4$ squares in it, which means he has $\displaystyle 12$ students with pets ($\displaystyle 3\times4=12$). Ms. Hen's bar has $\displaystyle 2$ squares in it, which means she has $\displaystyle 6$ students with pets ($\displaystyle 3\times2=6$). 

To find the total in both classes we add. 

$\displaystyle 12+6=18$

Each square represents $\displaystyle 3$ students. Ms. Smith's bar has $\displaystyle 5$ squares in it, which means she has $\displaystyle 15$ students with pets ($\displaystyle 3\times5=15$). Ms. Hen's bar has $\displaystyle 2$ squares in it, which means she has $\displaystyle 6$ students with pets ($\displaystyle 3\times2=6$). 

To find the total in both classes we add. 

$\displaystyle 15+6=21$

Each square represents $\displaystyle 7$ students. The first grade bar has $\displaystyle 4$ squares in it. That means we can take $\displaystyle 7\times 4$ to find our total. 

$\displaystyle 7\times4=28$

Each square represents $\displaystyle 7$ students. The second grade bar has $\displaystyle 5$ squares in it. That means we can take $\displaystyle 7\times 5$ to find our total. 

$\displaystyle 7\times5=35$

Each square represents $\displaystyle 7$ students. The third grade bar has $\displaystyle 2$ squares in it. That means we can take $\displaystyle 7\times 2$ to find our total. 

$\displaystyle 7\times2=14$

Each square represents $\displaystyle 7$ students. The first grade bar has $\displaystyle 4$ squares in it, which means there are $\displaystyle 28$ students who have a sibling ($\displaystyle 7\times4=28$).The third grade bar has $\displaystyle 2$ squares in it, which means there are $\displaystyle 14$ students who have a sibling ($\displaystyle 7\times2=14$). 

To find the difference we subtract. 

$\displaystyle 28-14=14$

Each square represents $\displaystyle 7$ students. The first grade bar has $\displaystyle 4$ squares in it, which means there are $\displaystyle 28$ students who have a sibling ($\displaystyle 7\times4=28$).The fourth grade bar has $\displaystyle 3$ squares in it, which means there are $\displaystyle 21$ students who have a sibling ($\displaystyle 7\times3=21$). 

To find the difference we subtract. 

$\displaystyle 28-21=7$

Each square represents $\displaystyle 7$ students. The fourth grade bar has $\displaystyle 3$ squares in it, which means there are $\displaystyle 21$ students who have a sibling ($\displaystyle 7\times3=21$).The third grade bar has $\displaystyle 2$ squares in it, which means there are $\displaystyle 14$ students who have a sibling ($\displaystyle 7\times2=14$). 

To find the difference we subtract. 

$\displaystyle 21-14=7$

Each square represents $\displaystyle 7$ students. The second grade bar has $\displaystyle 5$ squares in it, which means there are $\displaystyle 35$ students who have a sibling ($\displaystyle 7\times5=35$).The third grade bar has $\displaystyle 2$ squares in it, which means there are $\displaystyle 14$ students who have a sibling ($\displaystyle 7\times2=14$).

To find the difference we subtract. 

$\displaystyle 35-14=21$