SAT Question 9823f279: Right Triangles And Trigonometry answer and explanation with Desmos solution

An isosceles right triangle has a right angle and two legs of equal length. In the triangle shown, one angle is a right angle and the two legs each have a length of $15$. Thus, the given triangle is an isosceles right triangle. In an isosceles right triangle, the measures of the two non-right angles are $45^{\circ }$. It follows that the value of $x$ is $45$.