Question 26017
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