so what you do is distribute the invisible -1 in front of the second parenthasees set and add like terms
remember
x^2+3x^2=4x^2
-(3+4x-x^2)=-1(3x+4x-x^2)=-3-4x+x^2
so first distribute the negative 1
-1(-4f^2-3f+6)=4f^2+3f-6
add
12f^2-9f+15+4f+3f-6
group like terms
12f^2+4f^2-9f+3f+15-6
add
16f^2-6f+9
aswer is 16f^2-6f+9
