Isn't this Interest-ing?
Some minor changes I would recommend:


Replace the magic numbers with symbolic constants. It will be easier to
modify your program when these rates change. 

constexpr long double ANNUAL_MORTGAGE_COST = 0.03; 
constexpr long double LOAN_PRINCIPAL_PLUS = 0.06;
constexpr long double MARGINAL_TAX_RATE = 0.35;

Instead of doing the calculations inside main() I would suggest
putting it into a function. 

long double totalPayment(long double housePrice, long double downPayment){

    long double loanBalance = housePrice - downPayment;
    long double interest = LOAN_PRINCIPAL_PLUS * loanBalance;
    long double annualMortgage = (ANNUAL_MORTGAGE_COST * loanBalance) + interest;
    long double savings = MARGINAL_TAX_RATE * interest;
    long double totalCost = annualMortgage - savings;

    return totalCost;
}