\(\cos \theta = 0.5 \) implies an angle of 60 deg, but the negative sign means that the angle lies in the second or third quadrant, (where cosine is negative).
So, \(3x=120 \text{ or } 240 \text{ deg, } \)or, more generally 120 \(\pm\) 360k or 240 \(\pm\) 360k deg, where k is any integer.
Dividing by 3,
\(x = 40 \pm 120k, \text{ or }80 \pm 120k.\)
So,
\(x = \dots -80, 40, 160, 280, \dots\)
or
\(x = \dots -40, 80, 200, 320, \dots .\)
.