Question 946208
First off name the variables.
{{{H}}} - length of the page
{{{W}}} - width of the page
Write down what you know in algebraic language.
1.{{{H=2W}}}
The type doesn't take up the whole page because of the margin. 
Remember margin on top and bottom, margin on right and left. 
So the printed area is, 
2.{{{(H-6)(W-6)=36}}}
Substitute from eq. 1 into eq. 2,
{{{(2W-6)(W-6)=36}}}
Looks like the quadratic equation you need. 
Now solve.
{{{2W^2-12W-6W+36=36}}}
{{{2W^2-18W=0}}}
Solve for {{{W}}} then go back and solve for {{{H}}}.
Repost if you get stuck. 
Hint : Factor out W and solve.