The rule for a 90° rotation about the origin is to convert your (x, y) to (y, -x). Example: point A of your shape on the graph lies at (5, -9). To rotate this 90° clockwise, we would flip the X and Y values and make x value negative (or positive if it’s already negative). We would get (-9, -5). Example #2: (2, 4). -> (4, -2).
Note: this is clockwise. Counter clockwise would be converting your (x, y) to (-y, x). Basically the same as before except this time your Y value is the one changing negative (or positive if it’s already negative), rather than your x.