Problem for today

Either equation can be easily rearranged for $y$ : $\large 12+y=21 \xRightarrow[]{\text{subtract } 12} y = 21-12 = \boxed{9}$

To verify this, you can do the same to the other one: $\large 12-y=3 \xRightarrow[]{\text{add } y} 12=y+3 \xRightarrow[]{\text{subtract } 3} y = 12 -3 = \boxed{9}$

Thus, the answer is $\boxed{9}$

12 + y = 21

Subtract 12 from both sides.

12 - 12 + y = 21 - 12

y = 9

12 - y = 3

12 - 3 = y

9 = y

$12+\boxed{9}=21$ $12-\boxed{9}=3$

First, find an easier way. If 12+ =21 and 12- =3 what is ? Try 9 because, 12+9=21 then 12-9=3 right? Yes, so 9 is the right answer. :) YaY Can you find another way? I hope you can.