FCFactorOut[exp, pref] factors out pref out of exp. This is often needed to bring exp into a particular form that Mathematica refuses to give.
FCFactorOut[(a + 3 b), 3 b]3 b \left(\frac{a}{3 b}+1\right)
FCFactorOut[(a + 3 b), 3 b, Head -> hold]3 b \;\text{hold}\left(\frac{a}{3 b}+1\right)
FCFactorOut is also an option of Collect2
x^2 + 6 y
Collect2[%, {x, y}, FCFactorOut -> 3]x^2+6 y
3 \left(\frac{x^2}{3}+2 y\right)