Question 809625
L for length
w for width


{{{L = 6w}}} and {{{2(L+w)=42}}} directly from the description.


Substituting for L in the perimeter equation, {{{2((6w)+w)=42}}}
{{{12w+2w=42}}}
{{{14w=42}}}
{{{highlight(w=3)}}} meaning that {{{highlight(L=18)}}}


Your mistake was from:
{{{2(6*w)+2w=42}}}
Which should have gone,
{{{2*6w+2w=42}}}
{{{12w+2w=42}}}
{{{14w=42}}}, and as shown already...