In the reaction +n2 -> 2 nh3, what coefficient should be placed in front of h2 to balance the reaction?

In order to balance this equation you need to count each element and how many of the individual elements are in the equation.
_H2+N2=2 NH3
You multiply the 2 (Which is the coefficient) by the 3 (which is the subscript) This would equal 6 which indicated there are 6 hydrogen atoms on the right side so the left side should also have 6 hydrogen atoms

The missing coefficient on the left side must multiple the 2 to become 6 hydrogen
Answer=3