Kenneth S. answered • 01/16/17

I unveil the mysteries and secrets of trigonometry & you'll love it.

Make a little sketch showing the vector in 'standard position'; its magnitude is √58 (applying Pythagorean theorem).

angle θ can be determined as arctan (3/(-7)0 but you will have to make the usual adjustment that is needed since arctan results are in quadrant IV for negative arguments, and quadrant I for positive arguments; i.e. be sure your final angle θ is in quadrant II.