Question 939757
The very same example problem has been asked and answered a few times already.


w for width, L for length;

Given is L=2w, and using A for area, A=72.


Formula for area using the assigned variables, wL=A.


Summarizing the list of equations, purely in symbols:
L=2w
wL=A
-
KNOWN:  A=72
UNKNOWN:  w and L


Solution Process:
{{{wL=A}}}
{{{w(2w)=A}}}
{{{w*2*w=A}}}
{{{w^2*2=A}}}
{{{w^2=A/2}}}
{{{highlight(w=sqrt(A/2))}}}


Your choice if you want to rationalize the denominator, but knowing that {{{A/2}}} will be a whole number here, rationalizing does not matter much.


{{{L=2w}}} means now {{{L=2*sqrt(A/2)}}}.


Plug-in the given value and evaluate w and L.