31-9t^2=0\\\\(\sqrt{31}-3t)(\sqrt{31}+3t)=0\\\\ \sqrt{31}-3t=0 \ \ \ or \ \ \ \sqrt{31}+3t=0 \ \ |\ subtract\ \sqrt{31}\ to\ both\ sides      \\\\ -3t=-\sqrt{31}\ \ | \ divide \ both \ sides\  by\  (-3 )\ \ \ or \ \ \3t=-\sqrt{31}\ \ | \ divide \ both \ sides\  by\  3 \\\\t=\frac{\sqrt{31} }{3}\ \ \ or \ \ \ t=-\frac{\sqrt{31}}{3}    