Solution
We have the following function given:
[tex]f(x)=\frac{x+21}{x-7}[/tex]For this case we need to interchange x by y and we have:
[tex]x=\frac{y+21}{y-7}[/tex]And we need to solve for x and we have:
[tex]x(y-7)=y+21[/tex][tex]xy-7x=y+21[/tex][tex]xy-y=7x+21[/tex]Solving for y we got:
[tex]y(x-1)=7x+21[/tex][tex]y=\frac{7x+21}{x-1}[/tex]