The solubility of aluminum hydroxide in a buffer comprised of 0.100 M formic acid and 0.100 M sodium formate depends on the pH of the buffer solution. Aluminum hydroxide is amphoteric and its solubility varies with pH. At lower pH values, aluminum hydroxide is more soluble, while at higher pH values, it becomes less soluble. I would need more information about the pH of the buffer solution to provide a specific solubility value.