You have to set up 2 proportions.
If the 6.9 goes with the 5 then the proportion will look like this.
6.9/5 = x/6 Cross multiply
6 * 6.9 = 5x
41.4 / 5 = x
x = 8.28 or 8.3
If on the other hand the 6.9 goes with the 6 then the second proportion is
6.9/6 = x/5
6.9 * 5 = 6X
34.5 / 6 = x
x = 5.75
Answer: D <<<<<<<<======
X=21
This is because both of those angles are equal, so you would divide 42 by 2 and get 21