Question 456912
Ok, if you are saying your {{{f(x) = (4x^2-x^2)/x^2}}}, then this reduces to {{{f(x) = 3x^2/x^2 = 3}}} or f(x) = 3 or y = 3.

To find inverse, we replace y with x and solve for y, but this has no x so the inverse is simply x = 3. Both are lines.

If you are saying your {{{f(x) = 4x^2 - x^2/x^2}}}, then this reduces to f(x) = 4x^2 - 1 or y = 4x^2 -1. 

Now, replace x with y and solve for y:

x = 4y^2 - 1 --> 4y^2 = x + 1 --> y^2 = (x + 1)/4 and

y = {{{ +- sqrt((x+1)/4)}}}

But I think you really didn't write your equation correctly.