Consider that angle a is enclosed in a right triangle, then, you can use the tangent function to find a, as follow:
tan a = (opposite side)/(adjacent side)
based on the given information on the figure, the lengths of the adjacent side and opposite side are:
adjacent side = x = 9.91 in
opposite side = (y - z)/2 = (7.26 in - 3.64 in)/2 = 1.81 in
where it has been taken into account that the expression y-z is the length of two opposit sides (two triangles), and becasue of that, such expression is divided by 2.
Replace the values of adjacent and opposite sides into the formula for tan a:
tan a = (1.81 in)/(9.91 in)
tan a = 0.182
next, to find a, apply tan⁻¹ both sides of the previous equation:
tan⁻¹(tan a) = tan⁻¹(0.182)
a = tan⁻¹(0.182)
a = 10.35°
The measure of angle a is 10.35°, or
10.35° = 10° + 0.35°
consider 1° = 60', then
0.35°·(60'/1°) = 21'
Hence:
10.35° = 10° + 0.35° = 10° 21'
The measure of angle a is 10° 21'