The inverse of a function is when you switch the variables and solve for y again. For example, you function 
[tex]y =  \sqrt{2x - 3}  + 2[/tex]
[tex]x =  \sqrt{2y - 3}  + 2[/tex]
Now solve for y
[tex] \sqrt{2y - 3}  = x - 2[/tex]
[tex]2y - 3 =  ({x - 2})^{2} [/tex]
[tex]2y  =  {(x - 2)}^{2}  + 3[/tex]
[tex]y =  \frac{{(x - 2)}^{2} + 3}{2}[/tex]