Correct. First factor out $\displaystyle-{16}\usepackage{cmbright}\Huge\$ : $\displaystyle-{16}{\left({{t}}^{{2}}-{400}\right)}={0}\usepackage{cmbright}\Huge\$

$\displaystyle-{16}{\left({t}-{20}\right)}{\left({t}+{20}\right)}={0}\usepackage{cmbright}\Huge\$

$\displaystyle{t}-{20}={0}\usepackage{cmbright}\Huge\$ or $\displaystyle{t}+{20}={0}\usepackage{cmbright}\Huge\$

$\displaystyle{t}={20}\usepackage{cmbright}\Huge\$ or $\displaystyle{t}=-{20}\usepackage{cmbright}\Huge\$

20 is the solution, because time cannot be negative.