[tex]4x^2-81>0\\\\(2x)^2-9^2>0\\\\(2x-9)(2x+9)>0\\\\x_1=\frac92\,,\quad x_2=-\frac92\,,\qquad a>0\\\\x\in(-\infty\,,\,-\frac92)\cup(\frac92\,,\ \infty)[/tex]
