You may have found the correct value of k, but then instead of multiplying the product of the 35 and 20 by $\displaystyle\frac{{1}}{{2}}\usepackage{cmbright}\Huge\$ , you may have multiplied it by 2. The correct calculation would be $\displaystyle{y}={\left(\frac{{1}}{{2}}\right)}{\left({20}\right)}{\left({35}\right)}\usepackage{cmbright}\Huge\$ . The correct answer is 350 square centimeters.