In what direction is a line containing the points (5, 9) and (-5, -5) going

y=mx+b, where m is the direction of the line.
To find m we will use (y2 - y1)/(x2-x1) = m, (-5 - 9)/(-5-5) =-14/-10.
Because it is a negative number over a negative number we know that the direction is in the third quadrant. ie. between 180 and 270 degrees. or 1pi and 1 1/2 pi rad.
Now we use atan(-14/-10) = 234.46deg

Your calculator will give you 54.46 degress but because we know the line direction is in the third quadrant we add 180 degrees to get the final answer.