1) Mg+N2---->Mg3N2
3Mg+N2----->Mg3N2
I really don't know how to do this one..

2) Cu^2+ + Al----->Cu+Al^3+
2e+Cu^2+ ----> Cu X3
Al------>Al^3+ + 3e X2
6e+3Cu^2+ -----> 3Cu
2Al----->2Al^3 + 6e
6e+3Cu^2+ + 2Al ------------------> 3Cu+2Al^3 + 6e
3Cu^2+ + 2Al ------------------> 3Cu+2Al^3
I am not sure about this one either..

3) H2+Ag^+ ---->Ag+H^+
H2+Ag^+----------->Ag+2H^+
H2----->2H^+ +e
e+Ag^+----------->Ag
H2+e+Ag^+----------->2H^+ +e+Ag
H2+Ah^+ ------------> 2H^+ +Ag

I am not sure about all of above questions..
So, Please help me Thanks

1 & 2 are correct.

For 3, you missed something:

H_2 \rightleftharpoons 2H^+ + 2e^-

Now, you should be able to correct it.

Anyway, checking the charges on the right and left sides from your given answer, you have +1 on the left and +2 on the right. :)

So, I think number 3 is going to be
2H2 + Ag^+ ------------------------> 2H^+ + Ag

Thanks!

Hmm, you have 4 H on one side and only two on the other...

Ok, put one equation above the other like this:

Ag^+ + e^- \rightleftharpoons Ag
H_2 - 2e^- \rightleftharpoons 2H^+

Multiply the first one by two and add up, giving:

2Ag^+ + H_2 \rightarrow 2Ag + 2H^+