You may have found the correct value of k, but then chose the two measurements whose sum is 84. However, the numbers need to satisfy $\displaystyle{y}=\frac{{k}}{{x}}\usepackage{cmbright}\Huge\$ , an inverse variation equation. For the first triangle, $\displaystyle{12}=\frac{{k}}{{7}}\usepackage{cmbright}\Huge\$ , so $\displaystyle{k}={84}\usepackage{cmbright}\Huge\$ . Substitute 50 and 34.

$\displaystyle{50}=\frac{{k}}{{34}}\usepackage{cmbright}\Huge\$ does not yield $\displaystyle{k}={84}\usepackage{cmbright}\Huge\$ , so this is not an inverse relationship.