The angle of a straight line is 180°
so you can add both angle sides and equate it to 180°
(6x - 10) + (6x + 10) = 180
join like terms
6x + 6x -10 + 10 = 180
12x = 180 (-10 and +10 cancels out)
[tex] \frac{12x}{12} = \frac{180}{12} [/tex]
(12 and 12 cancels out)
x = 15
Your answer will the the first option
a) x = 15