Who knows how to do this 8 cos x+3=0 ? ?

You want to find the measure of angle x.  Rearrange the given 8 cos x+3=0 as 8 cos x= -3, or cos x = -3 / 8, or cos x = -0.375.  

Use the arccos function (inverse cosine) to find the value of x in radians and in degrees.

Since the cosine function is negative only in Quadrants II and III, focus on the following possible ranges:  Quadrant II:  90 < x < 180 and
Quadrant III:  180 < x < 270.

Example:  using my calculator to obtain the inverse cosine of -0.375, I got x = 1.96 radians.  Convert this into degrees if desired.  Draw a ray showing this angle.  Reflect this ray across the horiz. axis to obtain the angle in Q III.

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