let x be the width and y the length of the printing area. Then the size of the poster is:
You know that . Plug in these values into the equation of A:
. You get the extreme (minimum or maximum) value if the 1st derivative of A equals zero:
. Solve the equation
for x. I've got . Use this value to calculate y. I've got
And now you can calculate the length and the width of the complete poster.