Answer:
c = 7°
Explanation:
You want the value of c given that angles in a triangle are marked 86°, 31°, and 9c.
Angle sum theorem
The angle sum theorem tells you the sum of angles in a triangle is 180°. Using given angle values, this means ...
86° +31° +9c = 180°
117° +9c = 180° . . . . collect terms
9c = 63° . . . . . . . subtract 117°
c = 7° . . . . . . . . divide by 9
The value of c is 7°.