Google/Youtube is your best friend. If you ask here, you'll just get trolled. [spoiler]Write out the reaction equation, make sure it's balanced (same numbers of the same elements on both sides). Calculate the amount of moles of each you have based on the mass given and the molecular mass. Check to see which is your limiting factor (in this case, since you need equal numbers of each element to make the final product, it's the element with the smaller number of moles). Then calculate how much that many moles of the MgO weighs (mass).[/spoiler]