Given the equation 2mg(s) + o2(g) → 2mgo(s) + 72.3 kj, is this an endothermic reaction?

No and when you try it, don't go anywhere near the flame. It is very exothermic which means it is very hot. It gives off heat. Exothermic = gives off heat.

If the energy is plus and on the right, the reaction is exothermic.
If the energy is minus and on the right, the reaction is endothermic.

If the energy is plus and on the left, the reactions is endothermic. It requires energy to happen.
If the energy is minus and on the left, the reaction is exothermic. It must give up energy to happen.

Memorize these four sentences. You must have this at your fingertips for this section of the course.