You subtracted the amount of water already in the pond, when you should add that to the amount supplied by the hose. The total amount of water is $\displaystyle{8}+{10}={18}\usepackage{cmbright}\Huge\$ gallons after one minute, $\displaystyle{16}+{10}={26}\usepackage{cmbright}\Huge\$ gallons after two minutes, or $\displaystyle{24}+{10}={34}\usepackage{cmbright}\Huge\$ gallons after three minutes. You need to find an equation that matches these numbers.