Question 101965
{{{ 4x^2 - 81 }}}

As {{{ a^2 - b^2 = (a + b)*(a - b) }}} so that
{{{ 4x^2 - 81 = (2x + 9)*(2x - 9) }}}

Solution to the equation is therefore:
{{{ (2x + 9) * (2x - 9) = 0 }}}
{{{ x[1] = -9/2 }}} or {{{ x[2] = 9/2 }}}

Cheers :)