Apostal service will accept packages only if the length plus girth is no more than 264 inches. (see the figure.) assuming that the front face of the package (as shown in the figure) is square, what is the largest volume package that the postal service will accept?

So.. .we know the front face of the package is a square, that means the width and height are the same value, notice the picture below

thus... .if the length = l = 264 - 4x

well, the volume is length*width*height

thus   

so hmm, take the derivative of V(x), zero it out, check the critical points for any minima in the interval of (0, 264), since neither the height or width can be 0 or 264, but in between