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?

I assume you mean "the others." A pentagon has 5 sides.

4s + s + 3 = 128

5s + 3 = 128

Subtract 3 from both sides.

5s = 125

Divide both sides by 5.

s = 25

s + 3 = ?