A box with an open top is to be constructed from a rectangular piece of cardboard with dimensions W = 14 in. by L = 20 in. by cutting out equal squares of side x at each corner and then folding up the sides.

I found the function that models the volume V of the box
V(x) =4x^3-68x^2+280x

But I'm having trouble with finding the the values of x for which the volume is greater than 210 in^3. (Round your answer to three decimal places.)

AND finding the largest volume that such a box can have. (Round your answer to three decimal places.)