The length is 50. You solve it by making an equation out of the information given. the rectangle's length(l) is 3 times its width(w) and its perimeter(p) is 128. So in order to solve this you need an equation to find the perimeter, isolating w to find l. It'll look something like this:
(3w+8)+(3w+8)+w+w=128
?You combine all like terms turning it into:
2(4w+8)=128
8w+16=128
8w=112
w=14
you use the value of w to solve for l as follows:
l=w*3+8
l=14*3+8
l=42+8
l=50
