Here's one approach to your problem. Extend the line through (3, 5) and (2, 1) until it crosses the x -axis. We can find the slope of this line to be 4.
To determine the angle made with the x-axis, we take the arctan of 4 which is approximately 76 degrees (75.9637).
The angle formed at (3, 5) is the larger of the two angles in the parallelogram and is supplementary to the angle we just found.