If formula of sodium salt of an anion X is (Na_2X), then the formula of its aluminium salt would be

To determine the formula of the aluminium salt, we must first identify the charge of anion X. Sodium (Na) is a Group 1 metal that forms a cation with a +1 charge. Since the formula of the sodium salt is Na2X, the total positive charge from two sodium ions is +2; therefore, to maintain electrical neutrality, the anion X must have a charge of -2. Aluminium (Al) is known to form a cation with a +3 charge. Using the 'cross-over' or 'criss-cross' method to balance the charges between Al(+3) and X(-2), the magnitude of the charge on the cation becomes the subscript for the anion, and vice versa. This results in two aluminium ions (+6 total) balancing three X anions (-6 total), yielding the chemical formula Al2X3.

Sources

1. [1] https://www.bangor.ac.uk/sites/default/files/2020-10/chem-en-2019.pdf