Question

seventh term of (3x+y)^9

Answer 1

Pascal's triangle is what you want to use here, it's one of the most useful things in math. Just make a triangle like this: These tell you the coefficients on multiplication. For instance:

\[(a+b)^2= 1a^2+2ab+1b^2 \]\[(a+b)^3=1a^3+3a^2b+3ab^2+1b^3\] etc... see?