wut

g^{-1}(-7) = 30.

This question has been asked many tims recently. 
One of the others has proper answers.

Try to find what will produce -7 to get the answer to the question. If you think that x is smaller than -3, try various values of x. Will it work? Then try 21+x. This will be too high for any x in the given range. Then try the last equation. Once you have found what value of x will produce -7 for each equation, find which one is in the given range.

turns out the answer is 81