Your answer won't be "expressed in terms of pi", but it needs to be in radians.

Draw a diagram. Construct a right angled triangle in the third quadrant (where (-2, -5) is.

Use trigonometry (tan^-1) to find the acute angle below the neg x-axis ( this must be in radians). Add this angle to pi (to get the total angle anticlockwise from the pos x-axis). Then you'll have theta.