In order to find angle b, we have to know what supplementary angles.
Supplementary angles are two angles that add up to 180°, or in other words, a straight line.
Angle b and the 99 degree angle are supplementary due to them creating a straight line when combined, so they add up to 180°.
To find angle b, subtract 99 degrees from 180.
180 - 99 = 81
Angle b is equal to 81 degrees.