I'm lost. Could someone explain to me how I would approach these problems?

1a) Prove this equation combinatorially

$\displaystyle 2 \times 3^0 + 2 \times 3^1 + 2 \times 3^2 + ... + 2 * 3^{n-1} = 3^n - 1$

Next, solve this equation using ordinary base-10 numbers:

$\displaystyle 2 \times 10^0 + 2 \times 10^1 + 2 \times 10^2 + ... + 2 * 10^{n-1} = 10^n - 1$

1b) Let a and b be positive integers with $\displaystyle a > b$. Give a combinatorial proof of the identity $\displaystyle (a + b)(a - b) = a^2 - b^2$.