Answer: a = 64
Step-by-step explanation:
We use the difference of squares rule to say
(m-n)(m+n) = m^2 - n^2
So in this particular case,
(8z-9)(8z+9) = (8z)^2-(9)^2 = 64z^2 - 81
We can see that the 64 matches with the 'a' of the expression az^2-81, which must mean a = 64
Side note: This means b = 81