Correct. The information leads to the equation $\displaystyle{x}{\left({20}-{x}\right)}={84}\usepackage{cmbright}\Huge\$ . This becomes $\displaystyle{20}{x}-{{x}}^{{2}}={84}\usepackage{cmbright}\Huge\$ or $\displaystyle{0}={{x}}^{{2}}-{20}{x}+{84}\usepackage{cmbright}\Huge\$ . Use the Quadratic Formula:

$\displaystyle{x}=\frac{{-{\left(-{20}\right)}\pm\sqrt{{{{\left(-{20}\right)}}^{{2}}-{4}\cdot{1}\cdot{84}}}}}{{{2}\cdot{1}}}=\frac{{{20}\pm\sqrt{{{400}-{336}}}}}{{2}}=\frac{{{20}\pm\sqrt{{64}}}}{{2}}=\frac{{{20}\pm{8}}}{{2}}\usepackage{cmbright}\Huge\$

So $\displaystyle{x}={14}\usepackage{cmbright}\Huge\$ or 6. The baker charged $6 for one cake.