I had done my own working out to...

The problem

I had done my own working out to rewrite as the product of factors and found 2x(x+3) + -1(x+3)

I think I am misunderstanding something..

Could you tell me what I have done wrong? I used reverse FOIL...

I want to make a sign diagram from the factorised equation

Your result is correct since:

2x(x+3) + -1(x+3) = 2x(x+3) -1(x+3) = (x+3)(2x - 1) = (2x - 1)(x + 3)