$($Components of an annuity payment$)$ You take out a $30-$year mortgage for $$400,000$ to buy a new house. What will your monthly payments be if the interest rate on your mortgage is $7$ percent? Use a spreadsheet to calculate your answer. Now, calculate the portion of the $84$th monthly payment that goes toward interest and principal. Use five decimal places for the monthly interest rate in your calculations.

a$.$ Using a spreadsheet to calculate your answer, your monthly payments will be $ $($Round to the nearest cent.$)$