Make a sketch.
Draw the coordinate axes on a page: x-axis and y-axis.

It is yours coordinate plane.

Mark the point (16,12) in the plane. On the page, I mean. 
It has x-coordinate 16 and y-coordinate 12.

Connect this point with the origin of the coordinate system.
The distance to the point from the origin is  =  =  = 20.

Now,  =  = ,

and   =  = .


where  is what angle?  -  The standard position angle determined by the point (16,12).