Question 317481
{{{ d=kW }}}
Use your data to determine k.
{{{6=k(67)}}}
{{{k=6/67}}}
So then the equation is,
{{{d=(6/67)W}}}, when {{{W}}} is in lbs, {{{d}}} in inches. 
When {{{W = 81}}} lbs, then
{{{d=(6/67)*81=486/67 }}}or approximately
{{{d=7.25}}} inches.