Where a nonresident taxpayer earns, receives or acquires income from sources partly within and partly without this Commonwealth or engages in a business, trade, profession or occupation partly within and partly without this Commonwealth, and, as a result thereof or for other reasons that portion of the income derived from or connected with sources within this Commonwealth cannot readily or accurately be ascertained, the department shall by regulation prescribe uniform rules for apportionment or allocation of so much of such taxpayer's income as fairly and equitably represents income, derived from sources within this Commonwealth and subject to tax under this article.