Answer: Rotate 180 degrees around point B
Explanation:
Point A(-3,1) moves to A ' (1,1). The line through these two points is y = 1.
Point C(-3,3) moves to C ' (1, -1). The line through these two points is y = -x
These two lines intersect at point B(-1,1) which is the invariant point, aka fixed point. It is the center of rotation.
Notice how points A, B and A' are collinear (i.e. they are on the same line). The same can be said about points C, B and C'.
Because of these two sets of collinear points, it means we have a 180 degree rotation going on.
Check out the diagram below.
