When given f(x)=3x+12, find f(f^-1(x))

Trish receives $450 on the first of each month. josh receives $450 on the last day of each month. both trish and josh will receive payments for next four years. at a discount rate of 9.5 percent, what is the difference in the present value of these two sets of payments?

Idon't get undoing if its pass adding and subtracting so can someone ? x-2 over 5 = 18

If f(x) = 3x-2 and g(x) =2x+1 find (f-g)(x)