Find the factors that multiply to give 2n^2 and -9 but add to give 3n.
2n and n give 2n^2
-3 and 3 give -9
2n*3=6n ... (1)
n*(-3)=-3n ... (2)
(1)+(2)=3n
Group the factors that do not have an effect/interaction to each other from the above.
That is, 2n and -3, and n and 3
2n^2+3n-9=(2n-3)(n+3)