If x+3 is a factor of x^3-x^2-17x-15 what is another factor

The other two options are (x -5) or (x + 1).

The best way to go about finding this is by dividing. You can do this through synthetic division or by long division. 

After dividing by x + 3, you'll be left with x^2 - 4x - 5, which can be factored. To factor, always check factors of the last number that add to the middle number. The factors of -5 are either 1 and -5 or 5 and -1. Since the first pair add to the middle coefficient, use those in the parenthesis. 

X-3 x+3 are both factor and since x-3 isn't listed that would be your answer. Good luck