Question 240660
Use your calculator and find the "tan-1" button. Mine is the yellow above TAN (TI-83)

Depends on the calc you have, but you need to find the 'arctan' or 'tan -1'. 
Tangent takes degrees and turns them into a ratio. 
ArcTan takes a ratio (fraction) and turns it into degrees.

When you plug in tan-1(5/6), you'll get 39.8 degrees. Now you need to know which quadrant the plot it in to find 180-39.8