If you've found that (x + 2)(x - 3) is a factor of the original polynomial then you also know that:

(x^4 - 2x^3 + x^2 - 36)/[(x + 2)(x - 3)] = 0 for x not equal to -2 and 3. When you do the polynomial division you get:

x^2 - x + 6 = 0

I presume you can solve this from here?

-Dan