A vector starts at the point (0,0) and ends at (3,-3). What is the angle of the displacement?

It would be going at an angle down and right to the origin.

(3, -3)  forms  a  45 degree angle in the 4th quadrant =  [315° ]

It has a length of  sqrt [ 3^2  + (-3)^2 ] =   sqrt (18)  =   3sqrt (2)