Any element is a product of $\displaystyle h_1,...,h_n$ and since H is abelian (why?) we can even order that product in ascending index order:
$\displaystyle h_1^{\epsilon_1}\cdot...\cdot h_n^{\epsilon_n}\,,\,\,with\,\,\,\epsilon_i=0\,\,o r\,\, 1$
Well, in how many ways can you choose the first factor in such a product? In how many the second factor?....
Tonio