Answer:
 7f
 r = ----------- - 4
 z
Explanation:
I will assume that you meant:
 7f
z = ------------
 r + 4
Solve for r. Start by interchanging z and (r + 4):
 7f
r + 4 = -----------
 z
Now subtract 4 from both sides. This isolates r:
  7f
 r = ----------- - 4
 z