You need to find one root by substituting for x. Look at the constant 6. It has pairs of factors (1,6) and (2,3). One of these numbers may be a factor. Go for the smallest value first: 1. 1+2-5-6=-8 which is non-zero so x=1 is not a factor. Now try x=-1: -1+2+5-6=0 so x=-1 is a factor. Use synthetic division to find the quadratic factor:
-1 | 1 2 -5 -6
......1 -1 -1 6
......1 1 -6 | 0
The quadratic is x^2+x-6=(x+3)(x-2) so the factors of the cubic are -1, -3 and 2: (x+1)(x+3)(x-2).