Answer is: B₂O₃ + Mg → B + MgO.
B₂O₃ + 3Mg → 2B + 3MgO, balanced.
B₂O₃ - diboron trioxide. White, glassy solid compound.
B - boron. Metalloid, crystalline boron is and amorphous boron is a brown powder.
Mg - magnesium. Shiny, gray and solid metal.
MgO - magnesium oxide. White, hygroscopic solid powder.