The method described by tutor @alan3354 is the standard algebraic way to find the inverse of a function.
For relatively simple functions, an alternative method can get you to the answer faster using the concept that an inverse function "gets you back where you started".
The given function does this to the input, x:
(1) multiply by 7
(2) add 13
The inverse function has to do the opposite operations, in the reverse order:
(1) subtract 13
(2) divide by 7
ANSWER: The inverse function is
For practice, you should find the inverse by the standard algebraic method and verify that you get the same answer.