binomial expansion

In the expansion of (x^2+2)^n in descending powers of x,where n is a positive integer,the coefficient of the third term is 40.Find the value of n and the coefficient of x^4.

我識一半ng識一半= =

x^2n+(nC1(x^2)^x-1(2)+.....
the third terms = nC2 (2)^2x^2n-4

4(nC2)=40
n=5

之後唔識搵la
同埋唔明 ,有勞丫