Answer:
x = 13.8
Step-by-step explanation:
Theorem:
In a triangle, the angles opposite congruent sides are congruent.
The unlabeled angle also measures 5x.
The sum of the measures of the angles is 180.
5x + 5x + 42 = 180
10x + 42 = 180
10x = 138
x = 13.8
