Rotation of a shape
In order to rotate a shape 90 degrees counterclockwise about the origin our points P(x,y) becomes P'(-y,x).
Then,
for A (2, 1),
we have that x=2 and y = 1,
then our rotated point A',
must be A'(-y, x) = A'(-1, 2)
Applying the same operation to B and C we have:
B (1, 4) ⇒ B'(-4, 1)
because originally x = 1 and y = 4
C (4, 3) ⇒ C'(-3, 4)
because originally x = 4 and y = 3
Then we have:
A (2, 1) ⇒ B'(-1, 2)
B (1, 4) ⇒ B'(-4, 1)
C (4, 3) ⇒ C'(-3, 4)