To solve this expression, we can simplify each part separately and then multiply the results.

First, simplify the numerator:
28a^2 / 27x^3 = (4a / 3x)^2

Next, simplify the denominator:
-63x^4 / 140a = (-9x / 20a)^4

Now, we multiply the simplified numerator and denominator:
((4a / 3x)^2) ((-9x / 20a)^4)
= (16a^2 / 9x^2) (81x^4 / 160000a^4)
= (16 81) (a^2 x^4) / (9 160000 x^2 a^4)
= 1296 (x^2/x^2) (a^2/a^4)
= 1296 / (9 * 160000)
= 1296 / 1440000
= 9 / 10000

Therefore, the simplified expression is 9 / 10000.