The total amount Christian will pay is given by:
where P is the principal, r is the interes rate and t is the time. In this case we have that P=15000, r=0.07 and t=2. Then we have:
Hence he will pay $17,100 in total. Now, to find the monthly amount we divide the total by the number of months; in this case 24:
Therefore he will pay $712.50 each month.