Answer:
Equation: 139 + 3x + (5x - 10) + (10x - 9) + 3x = 540 (not simplified, see below for simplified)
x = 20
Step-by-step explanation:
If the sum of the angles is 540°, then that means you add them up to get 540°.
139 + 3x + (5x - 10) + (10x - 9) + 3x = 540
129 + 8x + (10x - 9) + 3x = 540
120 + 18x + 3x = 540
120 + 21x = 540
21x = 420
x = 20