As indi cited above, you may use the atanfull function to return the angle between two points:

angle# = atanfull(x2-x1, y2-y1)

Where x1/x2 and y1/y2 define the coordinates of your points.

The atanfull command returns a value between -180 and 180, so you'll have to format the resultant value if you require otherwise.

Example:

sync on
sync rate 0
` Define point 1 (center of screen)
x1 = screen width() / 2
y1 = screen height() / 2
while spacekey()=0
` Grab new position for point 2
x2 = mousex()
y2 = mousey()
` Calculate angle between points
angle# = atanfull(x2-x1, y2-y1)
` Display the angle
text 10, 10, str$(angle#)
line x1, y1, x2, y2
sync
cls
endwhile
end

*"so this is not team requests!"*
That's because I moved it