SAT Question e570a99c: Lines, Angles, and Triangles answer and explanation with Desmos solution

Choice A is correct. The sum of the measures of the three angles in a triangle is $180^{\circ }$. It's given that in $△JKL$, the measure of $\angle L$ is $118^{\circ }$. Let $x$ represent the measure of $\angle J$. It's also given that the measures of $\angle J$ and $\angle K$ are equal, so $x$ also represents the measure of $\angle K$. It follows that $x+x+118=180$, or $2x+118=180$. Subtracting $118$ from both sides of this equation yields $2x=62$. Dividing both sides of this equation by $2$ yields $x=31$. Therefore, the measure of $\angle J$ is $31^{\circ }$.