Binomial Theorem For Quadratic Equation

[Air]

Question:
Find the coefficient of x^5 in (1+x+x^2)^4.


Problem:
I have not come across expanding brackets which have x^2. I know how to apply the binomial theorem for (a+b)^n or (1+a)^n but have not come across (1 + ax + ax^2)^n. They are not explained in my textbooks so I was wondering if you could provide hints or redirect me to a useful link. Thanks.

 

[rock.freak667]

Just use b=x+x^2 and if you need to expand out b^2,b^3,b^4 just use the terms that will give you x^5