The first point is the center of the circular hole.

The second point is the intersection of the diagonals of the parallelogram.

Then cut along the straight line passing through the above two points, and you can divide the cake into two pieces of equal size.

Because this straight line passes through the center of the circle, it must divide the circle into two equal halves.

Because this straight line passes through the intersection of the diagonal lines of the parallelogram, the parallelogram must be divided into two equal halves.

So this straight line divides the cake into two pieces of equal size.