Since the quadrangle OECD is a rectangle and DE=OC=6, EH=4 and HD=2.

By HF//CD, so EF=2/3CE=2/3X.

FH = √ ((4× 4)-(2/3x )× (2/3x)) = √ (16-4/9xx) can be obtained from a right triangle.

According to the triangle area, s = y =1/2× x×√ (16-4/9xx).

And because x is not equal to 0 and 6.

Therefore, from the above function formula, we can know to choose a.