If you would like to know what is f(6), you can calculate this using the following steps:
f(n + 1) = f(n) - 8
f(1) = 100
f(2) = f(1) - 8 = 100 - 8 = 92
f(3) = f(2) - 8 = 92 - 8 = 84
f(4) = f(3) - 8 = 84 - 8 = 76
f(5) = f(4) - 8 = 76 - 8 = 68
f(6) = f(5) - 8 = 68 - 8 = 60
The correct result would be 60.