Switch the symbols x and y, and solve the resulting equation for y.
For example, find the inverse of the following function.
y = 1/4 * cos(2x)
Switch the variable symbols.
x = 1/4 * cos(2y)
Start solving for y by isolating the cosine function.
4x = cos(2y)
Get the y variable out from inside the function by taking the arccos of both sides.
arccos(4x) = 2y
Multiply by 1/2 to solve for y.
y = 1/2 * arccos(4x)
Now you try the same sort of thing with your exercise.
If you need more help, then please post any work that you've been able to accomplish and try to say something about why you're stuck.