The value that makes a quadrilateral QRST a parallelogram is 16.5
From the given parallelogram, the sum of adjacent side of the parallelogram is 180 degrees, hence:
4a - 8 + (8a - 10) = 180
Simplify
4a - 8 + 8a - 10 = 180
12a - 18 = 180
Add 18 to both sides
12a = 180 + 18
12a = 198
a = 198/12
a = 16.5
Hence the value that makes a quadrilateral QRST a parallelogram is 16.5