During one of the steps when you were subtracting, you made a mistake with a minus sign. You may have incorrectly gotten $\displaystyle-{10}{x}-{7}\usepackage{cmbright}\Huge\$ left over after the first subtraction. Or you may have correctly gotten $\displaystyle{10}{x}-{7}\usepackage{cmbright}\Huge\$ and made your mistake when you were subtracting the constants. The correct answer is 63.