Rotate 90 degrees counter clockwise about the origin

Rotate 90 counter clockwise about the origin 

$\begin{array}{rcl} \dbinom{x_p}{y_p}\cdot \begin{pmatrix} \cos{(90)} & \sin{(90)}\\ - \sin{(90)} & \cos{(90)} \\ \end{pmatrix}\\\\ = \dbinom{x_p}{y_p}\cdot \begin{pmatrix} 0&1\\ -1& 0\\ \end{pmatrix}\\\\ =\dbinom{-y_p}{x_p} \end{array}$