someone explain to me, help please

3. A debt of P 15,000 was paid for as follows: P 4,000 at the end of 3 months, P 5,000 at the end of 10 months, P 3,000 at the end of 17 months, and a final payment Fat the end of 21 months. If the rate of interest was 18% compounded quarterly, find the final payment F