Hello, mj.alawami!

I understand none of your last paragraph . . .

Find all the vlaues of x in that are solutions of the equation:

Attempt:

Since cosine is negative in the 2nd and 3rd quadrant

2nd quadrant

180 - 120 = 60

. . . . . . . . . . ??

x = 120°

3rd quadrant

No solution

We have: .

Then: .

Hence: .

Therefore: no solutions.