Set [tex]y = f^{-1}(x)[/tex] and [tex]f(y) = x[/tex]
Interchange the variables [tex]x[/tex] and [tex]y[/tex]
[tex]3y + 28 = x\\3y = x - 28\\y = \frac{x - 28}{3} \\\\f^{-1}(x) = \frac{x - 28}{3}[/tex]
