Question 576318
You are almost there. You are probably getting
{{{6r^2 = 85}}}
{{{r^2 = 85/6}}}
{{{r = sqrt(85/6)}}}
But your answer needs to be rationalized (it should not have a square root in its denominator
{{{r = sqrt(85/6) * sqrt(6/6)}}}
{{{r = sqrt(510) / 6 }}}