Basic mortgage calculator
"math.h" should be &lt;cmath&gt;.  The brackets are for system libraries whereas the quotes are for user-defined headers.  The .h ending is mostly for C.
You don't actually need return 0 at the end of main().  It will do this same return for you at the very end.
You should consider input validation, at least if you decide to expand on this outside of the class.  You could even allow the user the option to input via the command line, making this a bit more convenient.
You don't need those last two comments since they're already obvious.  Only use them where explanation is needed for you and/or the reader.
Consider having the calculation done in a function for better maintainability:

// add this to better explain that value
const int loan_term = 12;

double calculateMortgagePayment(int loan, double interest_rate)
{
    return (loan * interest_rate) / (1 - pow(1 + interest_rate, -loan_term));
}

int main()
{
    // ...

    // could also be const since value shouldn't change later
    const double mortgagePayment = calculateMortgagePayment(loan, interest_rate);

    // ...
}


I've taken into account@Conduit's advice regarding variable names.  I've also kept the double return type, which isn't really ideal for money.  There are resources online that explain more about that.