Don't be scared of negatives! Think of positive as moving to the right or forwards and negative as moving to the left or backwards. Every integer is a step. 2n-7n is two n steps forward and seven n steps back. So that's 5 n steps back: -5n. On the right we just have numbers: 20-5=15. -5n=15 is the same as 5n=-15. n is one fifth of 15 steps back, that's 3 steps back or -3.