First, simplify the expressions within the parentheses:
(5a-6у)^2 = 25a^2 - 60ay + 36y^2
(5a+3у)^2 = 25a^2 + 30ay + 9y^2

Substitute these back into the original expression:
(25a^2 - 60ay + 36y^2) - (25a^2 + 30ay + 9y^2)
= 25a^2 - 60ay + 36y^2 - 25a^2 - 30ay - 9y^2
= -90ay + 27y^2

Next, simplify the second part of the original expression:
(а^5-в^2)^2 = a^10 - 2a^5b^2 + b^4
(а^8-в^6)а^2-в = a^10 - a^2b^6 - b

Substitute these back into the original expression:
(a^10 - 2a^5b^2 + b^4) - (a^10 - a^2b^6 - b)
= a^10 - 2a^5b^2 + b^4 - a^10 + a^2b^6 + b
= -2a^5b^2 + a^2b^6 + b^4 + b

Therefore, the final simplified expression is:
-90ay + 27y^2 - 2a^5b^2 + a^2b^6 + b^4 + b.