Replace the f(x) with y and proceed to make x the subject:
y=1/4x-12
y+12=1/4x
To ÷1/4 on both sides, ×4 on both sides
4(y+12)=x
4y+48=x
Now swap y with x and replace the isolated x with f(x) to get the inverse function:
Remember that

So if
and
, then we know
.
Then use the Pythagorean identity to solve for cosine:

Answer:
28/5= 5 3/5 15/7= 2 1/7 21/4=5 1/4
Step-by-step explanation: