What is the orientation of u = (-4, -2)T ?
By plugging into the formula (and using a calculator):
arc tan( y/x ) = arc tan( -2/-4 ) = arc tan( 0.5 ) = 26.565°
But from the sketch, this is in the wrong quadrant. Study your sketch and use geometry to see that the answer must be (180 + 26.565 )° = 206.565°.
Say that (using a particular coordinate frame) you know that a vector is represented by (x, y)T.
You can calculate the two attributes of the vector:
orientation of (x, y)T = arc tan( y/x )
|(x, y)T| = √( x2 + y2 )
Sketch the vector r represented by (4, 5)T. Make an eyeball estimate of its length and direction. Then do the math to get the exact answer.