Question 312374
The perimeter of a rectangle is,
{{{P=2*(L+W)=168}}}
{{{L+W=84}}}
You also know,
{{{L=2W}}}
Substiute,
{{{2W+W=84}}}
{{{3W=84}}}
{{{highlight(W=28)}}}
Then 
{{{L=2W}}}
{{{highlight(L=56)}}}