There will be 12*15=180 cashflows.Monthly interest rate is 7%/12.
The first cashflow will be compounded 179 times,and the last cashflow will be at the end of 15 years and will not be compounded.
The future value,at the end of 15 years,will be:
180*(1+7%/12)^179 + 180*(1+7%/12)^178 + ...+ 180*(1+7%/12)^0
sum=180*((1+7%/12)^180-1)/(7%/12)=57053.21
or,you can input in excel:
=FV(7%/12,180,180,0)
and get the same number as above.