See Binomial Expansion.

Example

1. Square the first term: $(3x)^2 = 9x^2$
2. Double the product of both: Multiply $3x$ and $10$ together to get $30x$, then double it: $2 \cdot 30x = 60x$
3. Square the last term: $10^2 = 100$

\dots = $9x^2+60x+100