Question 834921
That is a good start.  CONTINUE!


Let's use L for length instead of the lower case "l", because it looks like a variable and not a digit.  


You know area A and you know perimeter P.  
You want to know length L and width w.


Equations for the system are as you saw, {{{2(L+w)=P}}} and {{{Lw=A}}}.


Solve for one unknown from one equation and substitute the formula into the other equation!  That is what is meant by, "CONTINUE!".  I will keep this generalized, really not caring about the actual values of P and A.


{{{2L+2w=P}}}
{{{2L=P-2w}}}
{{{L=(P-2w)/2}}}
-
{{{Lw=A}}}
{{{((P-2w)/2)w=A}}}
{{{(P-2w)w=2A}}}
{{{Pw-2w^2=2A}}}
{{{-2w^2-2A+Pw=0}}}
{{{-2w^2+Pw-2A=0}}}
{{{2w^2-Pw+2A=0}}}   --------- If the values for A and P make this factorable, then you can solve that way.  Otherwise, use the general solution for a quadratic equation:
{{{highlight(w=(P+- sqrt(P^2-4*(2)*(2A)))/(2*2))}}}
And find L from knowing value of w.