\frac{3x+12}{4}=\frac{3(x+4)}{4}=\frac{3}{4}(x+4)
\frac{8y-4}{4}=\frac{4(2y-1)}{4}=2y-1
\frac{-10a+15}{5}+\frac{12a-18}{6}=\frac{5(-2a+3)}{5}+\frac{6(2a-3)}{6}=-2a+3+2a-3=0
\frac{14x-21}{7}-\frac{64-16x}{8}=\frac{7(2x-3)}{7}-\frac{16(4-x)}{8}=2x-3-2(4-x)=2x-3-8+2x=
=4x-11