Unidentified metal M reacts with an unidentified halogen X to form a compound MX2. What is...?

First off, you know that you have 56 ml of X2, which translates to 0.00250 moles (divide 0.056 L of X2 by 22.41L/mol). The mass of X2 is (1.12 g - 0.72g) = 0.40g. Now you can calculate the molar mass of X2, which equals 0.40g/0.00250 moles = 160g/mol. Half of that number is the atomic mass of X = 80g/mol, so you're dealing with Br. You get 2 moles of MX per mole of X2, so there must be 0.0050 moles of MX, with a mass of 0.720g. Therefore the molar mass of MX = 0.720g/0.0050 moles = 144g/mole. Subtract 80 for the Br, and you get the mass of M = 64g/mole (= Cu).