It appears that you just subtracted the 1 from the 9. The ramp is the hypotenuse of a right triangle. You need to use the Pythagorean Theorem: $\displaystyle{{1}}^{{2}}+{{d}}^{{2}}={{9}}^{{2}}\usepackage{cmbright}\Huge\$ . The correct answer is $\displaystyle\sqrt{{80}}\usepackage{cmbright}\Huge\$ .