Question

cos(X)=-4/5

Answer 1

\[x=\cos^{-1}(\frac{ -4 }{ 5 }) \rightarrow x \approx 143.13^o\]

Answer 2

there are 2 possible answers

2nd quadrant where cos is negative and 3rd quadrant where cos is negative.

You have the 2nd quadrant answer above.

the 3rd quadrant answer is

\[180 + \cos^{-1} (\frac{4}{5}) \]

just evaluate it for the answer.