Create a  Rotation Matrix for 3 point x,y vectors

In general, the rotation matrix
$$\begin{bmatrix} \cos \theta & -\sin \theta \\ \sin \theta & \cos \theta \end{bmatrix} $$
rotates points in the $xy$-plane counterclockwise through an angle ? with respect to the positive x axis about the origin of a two-dimensional Cartesian coordinate system.

For $\theta=\frac{\pi}{2}$ (90 degrees) this matrix becomes 
$$\begin{bmatrix} 0 & -1 \\ 1 & 0 \end{bmatrix}. $$
If you want to to rotate a point $\begin{bmatrix} x \\ y \end{bmatrix} $  90 degrees with respect to the positive x axis about the origin, then the new point will be
\[\begin{bmatrix} 0 & -1 \\ 1 & 0 \end{bmatrix}\begin{bmatrix} x \\ y \end{bmatrix}= \begin{bmatrix} y \\ -x \end{bmatrix}.\]

Indeed a point $(x,y)$ becomes $(y,-x)$ after a 90-degree rotation. 

In this case you have a triangle with three vertices
$$(435, 500), (435, 456), (510, 543).$$

So after the rotation, the new coodniates will be 
\[(500, -435), (456, -435), (543, -510).\]