If you are studying this topic you must have come across the formula which in this case would say
100000 = Payment(1-(1.00708333)^(-240))/.00708333
payment = $867.82
Since the amount of interest will decline as the mortgage is gradually paid off, figuring the monthly payments is tedious.
Check this site to find the answer. Note that the numbers in your answer were correct, but the decimal point was in the wrong place.