Hello keadyjrWhat you have to do here is to let , and then make the subject of the formula.

So

, say.

So what now have is that (in other words, is the function that maps onto ), and . So maps back onto . Therefore is therefore the inverse of ; i.e.

Now it doesn't matter what letter is used for the parameter that's fed into a function, and it's usual to use rather than . So we would usually write:

I hope that clears it up OK.

Grandad