When you get to 2x^2-6x-5x+15, split the expression in the middle to 2x^2-6x and -5x+15 then factor out a common term from each which would be for the first 2x and the second -5. First works out to 2x(x-3) and second -5(x-3). so the factored expression is (x-3)(2x-5). Remember -5*-3=positive 15. not sure if I'm being real clear, but I hope that helps some.
2x^2-6x-5x+15 split 2x^2-6x and -5x+15, factor out common terms
(x-3)(2x-5) If you multiply both terms together, you get 2x^2-11x+15