For solving a question like this where you have to find a function for a particular value, just put the given number in place of the unknown variable i.e. x in this case. Lets solve this function now.
We are to solve for f(9), so we will put 9 in place of x:
f(x) = 72+4x
f(9) = 72 + 4(9)
f(9) = 72 + 36
f(9)= 108
so, your function for the value 9 is 108.
