Calculating solubility in water with other species present
Since the solution is acidic you have to consider $\ce{[OH- ]}$ concentration to be $10^{-7}$

Then solubility product: $$k_\text{sp} = \ce{[Al^3+ ] [OH- ]^3}$$

And if $x$ amount dissociated: $$k_\text{sp} = x \cdot (3x)^3$$

Then we have,

$$10^{-33} = 27 \cdot x^4$$

So $$x = 2.466 \cdot 10^{-9}~\mathrm{mol/L}$$