The following is the formula that can be used to complete that calculation:
Monthly Payment =
Monthly Interest Rate / (1  (1 + Monthly Interest Rate) ^{Payment Interval})) * Principal
Where
Monthly Interest Rate expressed as: Interest Rate / 100 / 12.
Payment Interval expressed as: Number of Years * Months of Year.
Principal is total amount of mortgage expressed without any comma separators or $ prefixing it.
An example for the above formula:
Rate: 7.75
Period: 360
Principal: $100,000
(7.75 / 100 /12) / (1  (1 + 7.75 /100 / 12) ^ (30 * 12) ) * 100000
The result of the above formula generates a value of 682.18. (Note that there is no formatting of the value.) The ^ is the symbol for an exponent that replaces the superscript in an actual formula.
In C++ you must declare variables for the rate, period, principal, and monthly payment. Each of these variables is prefixed with a data type. In this case the data types are limited to float for decimalbased numbers (interest rate and monthly payment) and int for whole numbers (payment interval and principal). NOTE: C++ in case sensitive so be sure the variable is the same in every instance.
Some of the Code:
Payment = ?? / (1  pow((1 + ??),  ?? * 12)) * ??;
cout << "Your monthly mortgage payment is $" << ?? << endl << endl;
[Remember to have #include $$
Test:
Rate

Period

Principal

Monthly Payment

3.50%

30 years

$350,000.00

1571.656

6.25%

15 years

$275,000.00

2357.913

7.325%

30 years

$468,000.00

3216.427*

5.50%

40 years

$725,000.00

3739.335

8.125%

30 years

$645,000.00

4789.107*

*Possible rounding error