Use the Binomial Theorem to write the first three terms of the binomial.
(x+2y)^10
(x+2y)^10
= C(10,0) x^10 (2y)^0 + C(10,1) x^9 (2y)^1 + C(10,2) x^8 (2y)^2 + ...
= x^10 + 20x^9 y + 180 x^8 y^2 + ...
check:
https://www.wolframalpha.com/input/?i=expand+(x%2B2y)%5E10