nth term in an expansion

[RUmolarman]

Heres a QR question from TOPSCORE 2....
What is the third term in the expansion of (x+3)^5
Answer= 90x^3


How do you get this??

 

[old-school]

In your college algebra book, look up pascal's triangle ie.
1
1 1
1 2 1
1 3 3 1 et. @ x^5 you hit 1 5 (10) a^2= 9(10)x^3