Hello out there.

I am fighting my way through Calc, hoping that some of the great help I've seen on here will save me gas going back and forth to the math help lab at school.

I need to find:

(f-1)’(a) for the function f and real number a

f(x)= x^3 - (4/x) with a being 6.

I'm obviously having an issue with the concept as it shouldnt be a hard problem, but I don't know how (other than guess/check) how to determine that "2" is the number I need to put into the inverse in order to get the answer, (1/13)

Thanks!