What, exactly, is the original question? You said "Find a possible equation for this third-degree polynomial if...
a = -3, b = -5, c = 1, and d = 5." But that gives no idea at all what "a", "b", "c", and "d" are.
mr fantastic guessed that you were using the form so you just put the given numbers in for the letters. If that is not correct then what does your text book or where ever you got this problem say a, b, c, and d are?