Well first try expanding a and b...
for example, you want to start by multiplying 4•n and 4•-3... what do you get? Well 4•n= 4n and 4•-3= -12. So A is 4n+(-12)-6n and then you want to further minimize this, so you would combine 4n and -6n. 4n+(-6n)= -2n. Therefore A= -2n+(-12). This is not the original equation. Do the same for B to finish solving.
