$-4{{t}^{2}}+20t=21\Rightarrow 4{{t}^{2}}-20t+21=0\Rightarrow \left\{\begin{matrix}   P=\frac{21}{4}  \\   S=\frac{20}{4}=5  \\ \end{matrix} \right.$   $\left| {{t}_{1}}-{{t}_{2}} \right|=\frac{\sqrt{\Delta }}{\left| a \right|}=\sqrt{\frac{{{b}^{2}}-4ac}{{{a}^{2}}}}=\sqrt{{{(\frac{b}{a})}^{2}}-4(\frac{c}{a})}=\sqrt{{{S}^{2}}-4P}=\sqrt{25-21}=\sqrt{4}=2$