This equation uses the rate at which the water fills up the pond, but you forgot that there are already 10 gallons of water in the pond. 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.