How to make trigonometry code more efficient

Question

I need help to make my code below more efficient, and to clean it up a little.

As shown in this image, x and y can be any point around the whole screen, and I am trying to find the angle t. Is there a way I can reduce the number of lines here?

Note: The origin is in the top left corner, and moving right/down is moving in the positive direction

o := MiddleOfScreenX - x;
a := MiddleOfScreenY - y;

t := Abs(Degrees(ArcTan(o / a)));

if(x > MiddleOfScreenX)then
  begin
    if(y > MiddleOfScreenY)then
      t := 180 + t
    else
      t := 360 - t;
  end
else
  if(y > MiddleOfScreenY)then
    t := 180 - t;

The code is in pascal, but answers in other languages with similar syntax or c++ or java are fine as well.

:= sets the variable to that value
Abs() result is the absolute of that value (removes negatives)
Degrees() converts from radians to degrees
ArcTan() returns the inverse tan

Answer 1

see this http://www.cplusplus.com/reference/clibrary/cmath/atan2/ for a C function.

atan2 takes 2 separate arguments, so can determine the quadrant.

pascal may have arctan2 see http://www.freepascal.org/docs-html/rtl/math/arctan2.html or http://www.gnu-pascal.de/gpc/Run-Time-System.html

o := MiddleOfScreenX - x;
a := MiddleOfScreenY - y;

t := Degrees(ArcTan2(o, a));