Molarity (M) is moles over liters (moles/liters), so convert grams to moles using the molar mass of MgO and then divide the moles of MgO over the liters given.
So it would look like this:
Step 1: 62.5 g MgO x (1 mole MgO/40.31 g MgO) = 1.55 mol MgO
Step 2: 1.55 mol MgO/1.5 L = 1.03 M MgO