Answer:
The answer to the solution can be found by calculating what "rotation by a certain angle" does to your x- and y axis:
90 deg This turns x into y, and y into -x. You can substitute these into your equation to get - x = 2y + 3 so y = - 1/2 * x - 3/2
180 deg : This turns x into -x, and y into -y. Your equation becomes - y = - 2x + 3 so y = 2x - 3
270 deg : This turns x into -y, and y into x. Your equation becomes x = - 2y + 3 so y = - 1/2 * x + 3/2