The question can't be answered. Given the interior angle sum, we know the number of sides of the polygon, and therefore the number of angles -- but we know nothing about how many of them are right angles.
So I will guess that the question was to find the number of angles, not the number of right angles.
I think the easiest way to get from the interior angle sum to the number of sides/angles is to add the 360 degree sum of the exterior angles; then, since the sum of each interior and corresponding exterior angle is 180 degrees, divide the resulting total by 180:
