SAT Question 410bdbe6: Lines, Angles, and Triangles answer and explanation with Desmos solution

Choice A is correct. The sum of the measures of the three interior angles of a triangle is $180^{\circ }$. Therefore, $31+2b+a=180$. Since it's given that $a=45$, it follows that $31+2b+45=180$, or $2b=104$. Dividing both sides of this equation by $2$ yields $b=52$.