You may have found the correct value of k, but then instead of multiplying the product of 30 and 18 by $\displaystyle\frac{{1}}{{2}}\usepackage{cmbright}\Huge\$ , you may have multiplied the product by 2. The correct calculation would be: $\displaystyle{y}={\left(\frac{{1}}{{2}}\right)}{\left({30}\right)}{\left({18}\right)}\usepackage{cmbright}\Huge\$