Find the monthly payment and estimate the remaining balance. Assume interest is on the unpaid balance.

Thirty year mortgage for $310,000 at 4.14%: remaining balance after 12 years.

Specifically what I am looking for is how to solve for the second half of the problem. Everything in the beginning makes sense.

I get that the monthly payment on the mortgage is $1505.12, but when I have to solve for the remaining balance I cannot figure it out. My friend told me that you solve for X and that X=144 in this problem. How do you find X!?