An angle is twice as big as it’s supplement. how big is the angle?

a parallel line has the same gradient, so the gradient would be 3 1/3

a perpendicular line has the negative inverse gradient of the original (eg. if the original gradient is 4/3 then the perpendicular gradient would be -3/4 (all you have to do is change the +/- sign and flip the fraction), so in your case the gradient of the perpendicular line would be -3/10 (since 3 1/3 = 10/3)