Required polynomial = x(x + 2)^3 = x(x^3 + 6x^2 + 12x + 8) = x^4 + 6x^3 + 12x^2 + 8x
You multiply each side by 5/2 because it's the reciprocal of 2/5 so 3 times 5/2 = 7.5 so y=7.5
If the drawing of your octagon (or whatever) has been separated into triangles, and one triangle's area<span> is labeled, then you do not need to know the apothem. Just take the </span>area<span> of that one triangle, and multiply by the number of sides in the original </span>polygon<span>.</span>