Height $\displaystyle={h}\usepackage{cmbright}\Huge\$ and Base $\displaystyle={h}-{6}\usepackage{cmbright}\Huge\$

$\displaystyle{20}=\frac{{1}}{{2}}{\left({h}-{6}\right)}{\left({h}\right)}\usepackage{cmbright}\Huge\$

$\displaystyle{20}=\frac{{1}}{{2}}{\left({{h}}^{{2}}-{6}{h}\right)}\usepackage{cmbright}\Huge\$

$\displaystyle{40}={{h}}^{{2}}-{6}{h}\usepackage{cmbright}\Huge\$

$\displaystyle{0}={{h}}^{{2}}-{6}{h}-{40}\usepackage{cmbright}\Huge\$

$\displaystyle{0}={\left({h}-{10}\right)}{\left({h}+{4}\right)}\usepackage{cmbright}\Huge\$

$\displaystyle{h}={10}\usepackage{cmbright}\Huge\$ or $\displaystyle{h}=-{4}\usepackage{cmbright}\Huge\$ Since the height is 10, the base is $\displaystyle{10}-{6}={4}\usepackage{cmbright}\Huge\$ .