To answer this question, we need to remember two theorems of parallelograms:
1. If a quadrilateral is a parallelogram, the two sets of its opposite angles are congruent:
2. The consecutive angles of parallelograms are supplementary (they sum 180 degrees):
Then, with this information, we have that:

And also, we have that the diagonal forms two congruent triangles, and the sum of internal angles of a triangle is equal to 180, then, we have:

Then, we have:

Then, using that the consecutive angles of parallelograms are supplementary (they sum 180 degrees), we have:

Thus, we have that the measure for angle a is:

In summary, we have that (all the measures in degrees):
m< a = 26
m< b = 57
m< c = 97