# Combinatorial proof help

$2 \times 3^0 + 2 \times 3^1 + 2 \times 3^2 + ... + 2 * 3^{n-1} = 3^n - 1$
$2 \times 10^0 + 2 \times 10^1 + 2 \times 10^2 + ... + 2 * 10^{n-1} = 10^n - 1$
1b) Let a and b be positive integers with $a > b$. Give a combinatorial proof of the identity $(a + b)(a - b) = a^2 - b^2$.