Business Finance Homework Help

My current mortgage has an annual interest rate of 7%. It will mature in 15 years. Originally it was a 30 year mortgage of the amount $200,000. Current mortgages with maturities of 15 years have interest rates of 4%. How much would I save per month in total before tax payments if I refinance the outstanding balance of my 7% mortgage with a new 15 year  4%mortgage? Payments are made monthly