Hey there! I'm happy to help!
When rotating a point 90 degrees clockwise about the origin, our original point (x,y) becomes (-y,x), because it is now at a negative y-value.
We see that our point P is at (1,2). We can use this rotation formula to find the coordinates of P' (the new spot where P is)/
(x,y)⇒(-y,x)
(1,2)⇒(-2,1)
Therefore, the coordinates of the point P' are (-2,1).
Have a wonderful day! :D