First off, use carats so its more clear that there's exponents (I'm using parentheses for simplicity):
(4*a^3*b^5)-(16*a^5*b^2)+(12*a^2*b^3)
It's best to divide the whole expression by 4a^2b^2 because it's the least common factor.
So we get (a*b^3)-(4*a^3)+(3b)
I hope this helped. It's been a while since I've done problems like these!