Quick question with the following addition

$\displaystyle

\frac{1(x-1)+1(x+2)}{(x+2)(x+3)(x-1)}=\frac{2x+1}{(x+2)(x+3)(x-1)}$

How are we able to combine the numerators in this case to produce 2x + 1? Usually when dealing with variables you are adding or subtracting like terms, but here I dont see it, if present.

Thanks