Well.. this is a bit diffucult for me to explain in english, but here goes:
Seeing as we've used the <=> symbol allthrough the inequation, the final line corrosponds the same function as the first line. In order to find the inverse function f^(-1)(x), we have to switch places of x and y.
By definition, the inverse function is the function that'll let you go from a value of y to a value of x - instead of a value of x to a value of y.
In other words, f-1(x) is f(x) mirrored by the axis y = x - as shown below:
Hope that explains it.. otherwise I'll be back to try to answer more questions