Rewrite in simplest radical form x56x16

I must assume that by "x56" you actually meant "x^56," or "x to the 56th power.

If that's the case, then x^56 * x^16 = x^ (56+16) = x^72, or "x to the 72nd power".

If at all possible, see a teacher regarding your "x56x16." It's very ambiguous.