Hi everyone. I'm confused with determining the general rule when a figure has been rotated NOT at 90, 180, 270, 360 degrees.

For example: In triangle ABC: A(4;4), B(4;1) C(2;0) has changed, C'(-1.5;1.5)... A'(-5,6;0) and B'(-3,6;3) NB - (not sure about A' and B' they approximations)

So how would i determine the general rule. How do i go about this? I know you use the formula:

(x;y) → (xCosθ - ySinθ; yCosθ + xSinθ)

Can i get some step by step help please... thnx