1._____Mg + _____N2 ---> _____Mg3N2

Question

2.

_____KNO3(s) ---> _____KNO2(s) + _____O2(g)

Answer 1

Mg + N2 ---> Mg3N2

Don't let these things intimidate you. The easy way to do this one (and it is easy) is to look on the right side. You need 3 Mg on that side. So place a three in front of Mg. For the right side you need 2N and you have N2 on the left. That's it.
3Mg + N2 ==> Mg3N2.
Now you do the second one.

2.

_____KNO3(s) ---> _____KNO2(s) + _____O2(g)