Abox 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.)