One of a pentagon's sides is 3 feet longer than the other. If the perimeter of the pentagon is 128m, how long is the longest side?

Your opening sentence is not clear.

Are you saying that 4 of the sides are equal and one is 3 feet longer?
If that is the case, let each of the equal sides be x
then the longer side is x+3, and

x + x + x + x + x+3 = 128
5x+3=128
5x=125
x=15
longer side is 18, the others are 15

25+25+25+25+25+28=128

minus one of those 25

Reiny did it right except i x=25 not 15

of course 125/5 = 25 , silly of me

To solve this problem, let's use algebra.

Let's assume the length of one side of the pentagon is x feet.
According to the problem, one side is 3 feet longer than the other side. So the length of the other side is (x + 3) feet.

The perimeter of a pentagon is the sum of the lengths of its sides. In this case, the perimeter is given as 128 feet.
So, the equation we can write is: x + x + x + x + (x + 3) = 128

Simplifying the equation, we have:
5x + 3 = 128
5x = 125
x = 25

Therefore, the length of the longest side of the pentagon is x + 3 = 25 + 3 = 28 feet.