The answer, going clockwise is -90. One leg of the angle is on the +x axis and the other is going down from the origin. The only other answer is 270 degrees. That angle is starting at the + x axis and goes anticlockwise. It has to travel to the dividing line between the 3rd and 4th quad.
-450 also works. Starting at 0 and going -360 clockwise leads you right back to the +x axis. -450 -(-360) leaves you with -90. Minus 90 degrees puts you on the line between the 3rd and 4th quadrant which is the same place 270 and - 90 will land.
Everything else takes you other places. 450 for example is the +y axis and that is related to 90 degrees not - 90.
