The equation can be factorized by inspection. The two numbers in the brackets must multiply together to make -165, so.....

If you find all the prime factors of 165, they are 3, 5, and 11. and note that 3x5 = 15

You also know that the sum of the two numbers in the brackets must be -4.

and.....

11x-15=-165

11+(-15)=-4

x^2-4x-165 = (x+11)(x-15)