In the reaction 3h2+n2--> nh3,what coefficient should be placed in front of nh3 to balance the reaction?

The best way to solve this is to list all the elements out and count them.

Count subscripts first (if there is no subscript then there is 1 atom)
Then multiply by the coefficients
To balance write a coefficient and multiply

3H2+N2--> NH3

H 2 x 3 = 6                H  3  (x2) = 6
N 2                            N  1  (x2) = 2

Your answer is 2 . . . to balance this equation you will need a coefficient of 2

Both models are effective because they show the valence electrons, and these valence electrons indicate the reactivity of an element.