A parallelogram has its sides determined by OA = (5,1) and OB = (-1,4), determine the angle between the two diagonals of this parallelogram.

Assuming I have the diagonal vectors going from bottom left to top right and bottom right to top left, the two vectors I'd end up with would be (4,5) and (-6,3), right?

From there do I use the dot product? I get a different answer than the book, which states it as 77.9