Question

how do you find the inverse of f(x)=3x^2-5

Answer 1

Let y=f(x)
Or, x=f^-1(y)
Right?

Answer 2

Now we can write
y=3x^2-5
x=?

Answer 3

Simply put, you just substitute y for x, basically flipping the equation; as an example:
y = 2x would invert to become x = 2y which is y = x/2.

In your situation you have the function notation f(y) = something with x, which can be written as y = some thing with x the simplify this process as the guy above or possible below me said.

That said, inverse function have a certain attribute which is, (f^-1(x) o f(x)) = (f(x) o f^-1(x)) = x, so you can check your work by looking making sure that evaluates properly.