You may have subtracted 311 from 282 instead of subtracting $\displaystyle{311}-{\left(-{282}\right)}\usepackage{cmbright}\Huge\$ . If a place is 282 feet below sea level, the elevation is $\displaystyle-{282}\usepackage{cmbright}\Huge\$ feet. The correct answer is that the house is 593 feet higher than the valley.