A box shaped block has a length equal to twice the width and the total surface area is 200cm^3. Find the dimensions of the maximum volume of the block:

This was my functions

Volume = [w(2w)] multiplied by (d)

SA = 2[w(2w)]+2(dw)+2(d(2w))

So

d = (200-4w^2)/(2(w+2w))

Is that right? How would you approach this question?