Two cables hang in a stadium. The first has direction ratios (2, 3, 6) and the second has direction…

The acute angle between lines with directions $(a_1, b_1, c_1)$ and $(a_2, b_2, c_2)$ has $\cos\theta = \dfrac{\lvert a_1a_2 + b_1b_2 + c_1c_2 \rvert}{\sqrt{a_1^2 + b_1^2 + c_1^2}\,\sqrt{a_2^2 + b_2^2 + c_2^2}}$. The lines are perpendicular when the dot product is $0$.