Hi Brisby55 - If you enter the beginning mortgage balance into your calculator as purchase price $577,500 * amount of leverage used 0.75 = $433,124 as PV, I/Y = 6%/12 = 0.5, N = 30 year loan * 12 months = 360, FV = 0 because it is fully amortizing, then CPT PMT = -2,596.80.
Then hit 2ND AMORT to bring up the mortgage amortization schedule. Since the property is sold 4 years into the mortgage you want the remaining mortgage balance as of 4 yr * 12 months = 48 periods. Enter this as both P1 and P2 and page through the amortization schedule which should give you the balance noted in the problem $409,799.