First, can some tell me if I did this right, and if not, tell me what I did wrong.

expand (x+4y)^5

I got: x^5 + 20x^4y + 26x^3y^2 + 74x^2y^3 + 261xy^4 + 1024y^5

Second, I've forgotten the formulas for exponential growth and decay, If someone could provide those I'd greatly appreciate it.