I'll presume you're looking for the angle which the vector makes with the x-axis.
We use our old friend tan for this.
tantheta=y/x
=>theta=tan^-1(y/x)
color(white)(=>theta)=tan^-1((-3)/4)=tan^-1(-0.75)
color(white)(=>theta)=-0.64
color(white)(=>theta)approx-36.67°
Because tan^-1 can only return angles between -90° and +90° (that is, in Q_"IV" or Q_"I"), we then check which quadrant the point (4,"-"3) is in, to confirm whether we can keep this answer or if we need to add 180°.
(4,"-"3) is in quadrant 4, and so the value given by tan^-1 is fine as it is.
The vector V=(4,"-"3) is at an angle of -36.67° to the positive x-axis.
