I need to find the matrix of reflection through line $y=-\frac{2}{3}x$

I'm trying to visualise a vector satisfying this. The standard algorithm states that we need to find the angle this line makes with x axis and the transformation matrix can be seen as ${R}_{\alpha}{T}_{0}{R}_{-\alpha}.$

I'm not sure how to proceed. I can't visualise the angle it makes with x axis. Is there a procedure to think about such reflections?

I'm trying to visualise a vector satisfying this. The standard algorithm states that we need to find the angle this line makes with x axis and the transformation matrix can be seen as ${R}_{\alpha}{T}_{0}{R}_{-\alpha}.$

I'm not sure how to proceed. I can't visualise the angle it makes with x axis. Is there a procedure to think about such reflections?