SAT Question 69f4bbdc: Lines, Angles, and Triangles answer and explanation with Desmos solution

Choice B is correct. The triangle shown is a right triangle, where the interior angle shown with a right angle symbol has a measure of $90^{\circ }$. It's shown that the other two interior angles measure $13^{\circ }$ and $a^{\circ }$. The sum of the measures of the interior angles of a triangle is $180^{\circ }$; therefore, $90+13+a=180$. Combining like terms on the left-hand side of this equation yields $103+a=180$. Subtracting $103$ from both sides of this equation yields $a=77$.