So, if I'm understanding you correctly, f(x)=x^2 divided by 3x+x, or
f(x) = (x^2)/(3x+x)
f(6) --> plug the 6 in to all values of x --> f(6) = (6^2)/(3×6 + 6) = 36/(18+6)
= 36/24 --> both are divisible by 12, so 36/12=3 and 24/12=2, now we have the reduced (simplified) answer:
f(6) = 3/2, or 1 1/2, or 1.5