First, we need to distribute each term in the first set of parentheses to each term in the second set of parentheses:

(5у)(12х) + (5у)(-6х) + (-7а)(12х) + (-7а)(-6х) + (5у)(12х) + (5у)(6х) + (-7а)(12х) + (-7а)(6х)

Now, simplify each term:

60ух - 30ух - 84ах + 42ах + 60ух + 30ух - 84ах - 42ах

Combining like terms, we get:

60ух - 30ух + 60ух + 30ух - 84ах + 42ах - 84ах + 42ах
= 120ух - 120ах

Therefore, the simplified expression is 120ух - 120ах.