COS A=\FRAC{({{b}^{2}}+{{c}^{2}}-{{a}^{2}})}{2bc}\] \[\Rightarrow \cos {{60}^{o}}=\frac{1}{2}=\frac{9+{{c}^{2}}-16}{2.3c}\]Þ \[{{c}^{2}}-3c-7=0\].