(a) Subject to § 171.1055, in apportioning margin, the gross receipts of a taxable entity from its entire business is the sum of the taxable entity’s receipts from:
(1) each sale of the taxable entity’s tangible personal property;
(2) each service, rental, or royalty; and
(3) other business.
(b) If a taxable entity sells an investment or capital asset, the taxable entity’s gross receipts from its entire business for taxable margin includes only the net gain from the sale.

(c) A combined group shall include in its gross receipts computed under Subsection (a) the gross receipts of each taxable entity that is a member of the combined group, without regard to whether that entity has a nexus with this state for the purpose of taxation.