SAT Question 36200a38: Lines, Angles, and Triangles answer and explanation with Desmos solution

Choice B is correct. The sum of the interior angles of a triangle is $180^{\circ }$. The measures of the two interior angles of the given triangle are shown. Therefore, the measure of the third interior angle is $180^{\circ }-70^{\circ }-50^{\circ }=60^{\circ }$. The angles of measures $x^{\circ }$ and $60^{\circ }$ are supplementary, so their sum is $180^{\circ }$. Therefore, $x=180-60=120$.