Can the perimeter be 18 and still the area is 42?

For a given perimeter, the greatest possible area is enclosed by a circle.

A circle with a circumference of 18 has a diameter of (18/pi) and a radius of (9/pi).

Its area is (pi R2) = (pi 92/pi2) = 81/pi = 25.78 (rounded)

So an area of 42 cannot be enclosed by a perimeter of 18.