SAT Question 6933b3d9: Right Triangles And Trigonometry answer and explanation with Desmos solution

No matter where point $W$ is on $\overline{RT}$, the sum of the measures of $\angle RSW$ and $\angle WST$ is equal to the measure of $\angle RST$, which is $90^{\circ }$. Thus, $\angle RSW$ and $\angle WST$ are complementary angles. Since the cosine of an angle is equal to the sine of its complementary angle, $\cos (\angle RSW)=\sin (\angle WST)$. Therefore, $\cos (\angle RSW)-\sin (\angle WST)=0$.