We need to find angle a in the figure.
We know that:
x = 9.342 inches
y = 6.692 inches
z = 2.952 inches
We can do so by finding the legs in the following triangle:
The adjacent leg is x. And the opposite leg is found by subtracting z from y, and then dividing the result by two (assuming the figure is symmetric):

Thus, we have:
