sev, do these dimensions seem correct. My calculations feel off. where is my mistake in the calculation?

A maling service places a limit of 78 in. on the combined length and girth of (distance around) a package to be sent parcel post. What dimensions of a rectangular box with square cross-section will contain the largest volume that can be mailed? (Hint There are two different girths.) The dimensions are x = 22 and y