Chemistry help?

4 NO molecules contain 4 oxygen atoms and thus can form 1 N₂O₄ with 2 N atoms left, which will form a N₂ molecule.

Therefore, the balanced equation is :
4 NO(g) → N₂(g) + N₂O₄(g)

(In the equation, there are 4 moles of gaseous molecules on the reactant side and 2 moles of gaseous molecules on the product side. When the reaction occurs, pressure decreases as the decreases in gaseous molecules.)