Arectangle's length is 8cm more than three times its width it the perimeter is 128 cm find the length

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