Answer
$a=45$
Work Step by Step
According to the Binomial Theorem, the 9th, that is, the ($8+1$)th term of $(z^2-y)^{10}$ is:
$_{10}C_8(z^2)^{10-8}(-t)^8=\frac{10!}{(10-8)!\times8!}z^4t^8=45z^4t^8$
$az^4t^8=45z^4t^8$
$a=45$
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