## Precalculus (6th Edition) Blitzer

The measures of the three interior angles are $75{}^\circ ,75{}^\circ ,30{}^\circ$.
By using angle sum property, the equation is obtained as follows: $x+2y=180{}^\circ$ (I) We know that one interior angle is y and one exterior angle is $3x+15$ and both lie on the same line; thus, their sum is $180{}^\circ$. And the equation is obtained as: \begin{align} & 3x+15+y=180{}^\circ \\ & 3x+y=165{}^\circ \end{align} (II) And to solve these equations, multiply equation (I) by $3$ and subtract equation (II): \begin{align} & 5y=375{}^\circ \\ & y=75{}^\circ \end{align} And to obtain the value of x, substitute the value $y=75{}^\circ$ in either of the equations: \begin{align} & x+2\left( 75 \right)=180{}^\circ \\ & x=30{}^\circ \end{align} Hence, the measures of the interior angles are $75{}^\circ ,75{}^\circ ,30{}^\circ$.