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{a}={21}\usepackage{cmbright}\Huge\$ and $\displaystyle{c}={29}\usepackage{cmbright}\Huge\$ , and then solve for b.