The perimeter is just adding all the sides together.
a.) 180 = 40 + 40 + x + x
180 = 80 + 2x Subtract 80 on both sides
100 = 2x Take 1/2 of 100 to get x
x= 50cm
b.) 1000 = 150 + 150 + x + x
1000 = 300 + 2x Subtract 300 on both sides
700 = 2x Take 1/2 of 700 to get x
x = 350
