In this problem, you're looking for the value of t, so that f(t) = 48. That means if you put t into 5t-7, you'll get 48. To do this set up and solve an equation for t by setting 48 equal to 5t-7:
Isolate the variable by adding 7 to both sides.
Divide by 5 on both sides to find t.
The answer is 11.