The inverse of the function f(x) = 1/2x + 10 is shown. h(x) = 2x – ? what is the missing value?

Take the f(x) and replace with y to get 

y = .5x + 10. 

Then flip the y and x to find the inverse.

x = .5y + 10

Now solve for the new y..

x - 10 = .5y

2x - 20 = y.

The missing value is 20.