You chose the two measurements whose sum is the same as the sum of the measurements of the prism given in the question. However, the numbers need to satisfy $\displaystyle{y}=\frac{{k}}{{x}}\usepackage{cmbright}\Huge\$ , an inverse variation equation. For the first triangle, $\displaystyle{36}=\frac{{k}}{{6}}\usepackage{cmbright}\Huge\$ , so $\displaystyle{k}={216}\usepackage{cmbright}\Huge\$ . Substitute 21 and 21.

$\displaystyle{21}=\frac{{k}}{{21}}\usepackage{cmbright}\Huge\$ does not yield $\displaystyle{k}={216}\usepackage{cmbright}\Huge\$ , so this is not an inverse relationship. The correct answer is 54 and 4.