The first big challenge here is setting up the function for Volume. By drawing a picture of the rectangle with the corners of length x cut out, we get a volume formula of:
V = (12-2x)*(20-2x)*x = 4x3-64x2+240x
The (12-2x) comes from the shorter side with two lengths of x cut out
The (20-2x) comes from the longer side with two lengths of x cut out
The factor of x comes from the size of the cuts giving the height of the box when the sides are folded up.
We then just need to set this function greater than or equal to 230:
4x3-64x2+240x >= 230
4x3-64x2+240x-230 >= 0
We can use a calculator or algebraic software like wolfram alpha to find the roots of the polynomial in the last step:
Approximately: x = 1.5067, 3.4584, 11.035
By looking at the graph, the part that we want is between the first two roots: [1.5067, 3.4584] and to the right of the third root: [11.035, infinity]
However, we have to keep in mind the physical limitations of the problem. Since the shorter side of the rectangle is 12, then the largest square we could cut out would have side length 6. This means we really just want the first interval above (not the one going to infinity)
Hope this is helpful!