It appears that you just subtracted the leg length from the hypotenuse. You need to use the Pythagorean Theorem: $\displaystyle{{a}}^{{2}}+{{b}}^{{2}}={{c}}^{{2}}\usepackage{cmbright}\Huge\$ .

Substitute the values $\displaystyle{b}={40}\usepackage{cmbright}\Huge\$ and $\displaystyle{c}={41}\usepackage{cmbright}\Huge\$ , and then solve for a.