Chapter 11 - 11.5 - The Binomial Theorem - 11.5 Exercises - Page 813: 51

$a=-6300000$

According to the Binomial Theorem, the 6th, that is, the ($5+1$)th term of $(2x-5y)^9$ is: $_9C_5(2x)^{9-5}(-5y)^5=\frac{9!}{(9-5)!\times5!}(2x)^4(-3125)y^5=(126)(16)x^4(-3125)y^5=-6300000x^4y^5$ $ax^4y^5=-6300000x^4y^5$ $a=-6300000$