There has to be a centre about which to rotate. If the centre is the origin, and you know the coordinates of the point, then (a, b) becomes (-b,a)

for example (3,4) becomes (-4,3)

otherwise, join the centre of rotation to your point to get a line. Use your protractor to measure 90 degrees clockwise/anticlockwie and make the length of the new line the same as the original.