Question 551327
For a rectangle, the perimeter, P, is given by:
{{{P = 2(L+W)}}} Substitute {{{L = 6x}}}, {{{W = 2x}}}, and {{{P = 90}}}
{{{90 = 2(6x+2x)}}} Simplify.
{{{90 = 16x}}} 
{{{x = 5.26}}}
The longest side is 6x, so...
{{{6x = 33.75}}}