The answer would be 99 degrees!
here is an explanation to help you understand:
the angles 81 degrees and b are supplementary angles. Supplementary angles mean that two angles have the sum of 180 degrees so you would need to find the x when you have a given already.
do an equation like this:
x + 81 = 180
here is how to answer the equation:
x + 81 - 81 = 180 - 81
x = 99
Here you only need to subtract 81 from both sides. in simple terms, do 180 minus the measure given.
i hope this helps!! i am doing this lesson right now so i know and if u need more help tell me in comments or message :)