[tex](8z-9)(8z+9)=az^2-b\ \ \ \ |add\ "z"\ to\ both\ sides\\\\(8z-9)(8z+9)+z=az^2\ \ \ \ |divide\ both\ sides\ by\ z^2\neq0\\\\\boxed{a=\frac{(8z-9)(8z+9)+z}{z^2}}\\\\use:\ (a-b)(a+b)=a^2-b^2\\\\a=\frac{(8z)^2-9^2+z}{z^2}\\\\use:\ (a\cdot b)^n=a^n\cdot b^n\\\\a=\frac{8^2z^2-81+z}{z^2}\\\\\boxed{\boxed{a=\frac{64z^2+z-81}{z^2}}}[/tex]
