Answer: 15
Step-by-step explanation:
Let's start by stating what we are given
The perimeter is 36
One side length is 12
And the other given side is 9
Finally we have an unknown side x
The value of all the sides added together is the perimeter.
So add all the sides
12+9+x=36
21+x=36
Subtract both sides by 21
21-21+x=36-21
x=15
