What is the inverse of the function
y = 3x2 ?
If you mean y = 3x^2 then there is no inverse, since there are two values of x for every value of y.
However,
y/3 = x^2
x = √(y/3)
so, f -1(x) = √(x/3)
that will give you one side of the parabola, and -√(x/3) will give you the other side.