To simplify this expression, we can first simplify each term individually and then combine them.

Starting with 28A^2:
28A^2 / 27X^3 = 28A^2 / (27X^3)

Next, simplify -63X^4:
-63X^4 / 140A = -63X^4 / 140A

Now, we can combine the two terms:
(28A^2 / 27X^3) • (-63X^4 / 140A)

To simplify, multiply the numerators together and the denominators together:
(28 -63)A^2X^4 / (27 140)AX^3

Simplify the numerator: -1764A^2X^4
Simplify the denominator: 3780AX^3

Final simplified expression:
-1764A^2X^4 / 3780AX^3

This can be further simplified by dividing both the numerator and denominator by 42:
-42A^2X / 90AX^2

Therefore, the final simplified expression is:
-7A / 15X