Hey there!
I assume you want this equation balanced.
Al₂(SO₄)₃ + Ca(OH)₂ → Al(OH)₃ + CaSO₄
Let's start by balancing the polyatomic ions, starting with SO₄.
There are 3 on the left and 1 on the right, so let's add a coefficient of 3 in front of the CaSO₄.
Al₂(SO₄)₃ + Ca(OH)₂ → Al(OH)₃ + 3CaSO₄
Now let's balance the polyatomic ion OH.
There are 2 on the left and 3 on the right, so let's put a coefficient of 3 in front of Ca(OH)₂ and a coefficient of 2 in front of Al(OH)₃.
Al₂(SO₄)₃ + 3Ca(OH)₂ → 2Al(OH)₃ + 3CaSO₄
Now let's balance Al.
There are 2 on the left and 2 on the right, so Al is already balanced.
Lastly, let's balance Ca.
There are 3 on the left and 3 on the right, so Ca is already balanced.
This means we're done! Here's the final balanced equation:
Al₂(SO₄)₃ + 3Ca(OH)₂ → 2Al(OH)₃ + 3CaSO₄
Hope this helps!