Look closely at your equation. For every 2 moles of Aluminum, you will get 3 moles of H2.... this is a (2:3) ratio.
If you want to make 6 moles of H2, you need to have TWICE the amount of the original 3 moles of H2... simply multiply the original moles of Aluminum (2) by two, as well, to maintain the same ratio.
2 x 2 = 4 moles of aluminum
This makes our ratio (4:6) which would simplify to (2:3), proving the answer to be true based on the balanced equation.
Hence, you need 4 moles of aluminum.