[tex]\textbf{3)}\\\\~~~~~3x^2\cdot 5 = 80\\\\\implies 15x^2 = 80\\\\\implies x^2 = \dfrac{80}{15}\\\\\implies x^2 = \dfrac{16}3\\\\\implies x = \pm\sqrt{\dfrac{16}3}\\\\\implies x = \pm \dfrac 4{\sqrt 3}\\\\\textbf{4)}\\\\~~~~~~(x-5)^2 = 81\\\\\implies x -5 =\pm\sqrt{81}\\\\\implies x -5 = \pm9\\\\\implies x = 5\pm9\\\\\implies x = -4,~ x = 14[/tex]
