To balance the equation, you need to make sure that the number of atoms of each element is the same on both sides of the equation.
The balanced equation for the reaction of aluminum selenide with water is:
Al2Se3 (s) + 6H2O (l) → 2 Al(OH)3 (s) + 3H2Se (g)
Aluminum selenide (Al2Se3) is a solid (s) and water (H2O) is a liquid (l). Aluminum hydroxide (Al(OH)3) is a solid (s) and hydrogen selenide (H2Se) is a gas (g).
Remember, it is important to check that the equation is balanced by counting the number of atoms of each element on both sides of the equation. In this case, there are 2 aluminum (Al), 3 selenium (Se), 12 hydrogen (H), and 9 oxygen (O) atoms on both sides of the equation, making it balanced.