Question 26021
Simple: the formula for perimeter of a rectangle is 2*(a+b), where a and b are the sides:
side b=8
side a=?

40=2*(a+8)  divide by two both sides and get:
20=a+8   subtract 8 from both sides and:
a=12   whoala!!!

now plug it in back to check the answer:

is 40=2*(12+8)??
40=2*20
40=40   you betcha!!!!
good luck