Magnesium is burned in oxygen to produce magnesium oxide, and the chemical formula is 2Mg+O2=2MgO. Why not Mg+O2=MgO? But the book is Mg+O2=MgO.

Law of conservation of mass: the total mass is conserved before and after the reaction. In fact, specifically, the types and qualities of elements remain unchanged, and the types and quantities of atoms remain unchanged. So 2Mg+O2=2MgO is right.

In Mg+O2=MgO, the number of oxygen atoms on the left and right sides is different. The book is printed wrong. Either your classmate made a mistake or you are asked to correct it in the book. In short, it must be wrong.

PS: the arrow is right! Arrow and equal sign are not easy to use in series. The former is a chemical reaction equation and the latter is a chemical reaction equation. In the future, you will learn the law of conservation of mass and the writing of chemical equations. It doesn't matter!