You appear to have started with the correct equation $\displaystyle{0}=-{16}{{t}}^{{2}}+{25}{t}+{5}\usepackage{cmbright}\Huge\$ , but when changing it to standard form, you made a sign mistake and solved $\displaystyle{16}{{t}}^{{2}}-{25}{t}+{5}={0}\usepackage{cmbright}\Huge\$ rather than the correct standard form of $\displaystyle{16}{{t}}^{{2}}-{25}{t}-{5}={0}\usepackage{cmbright}\Huge\$ .