\(\begin{align} & \begin{array}{*{35}{l}} \underset{x\to -\infty }{\lim}\,\left(\sqrt{4{{x}^{2}}+3}-\sqrt{4{{x}^{2}}+5x+1} \right) \\ =\underset{x\to -\infty }{\lim}\,\frac{\left(\sqrt{4{{x}^{2}}+3}-\sqrt{4{{x}^{2}}+5x+1} \right)\left(\sqrt{4{{x}^{2}}+3}+\sqrt{4{{x}^{2}}+5x+1}\right)}{\sqrt{4{{x}^{2}}+3}+\sqrt{4{{x}^{2}}+5x+1}} \\ \end{array} \\ & =\underset{x\to -\infty }{\lim}\,\frac{\left(4{{x}^{2}}+3 \right)-\left( 4{{x}^{2}}+5x+1\right)}{\sqrt{4{{x}^{2}}+3}+\sqrt{4{{x}^{2}}+5x+1}} \\ & =\underset{x\to -\infty }{\mathop{\lim}}\,\frac{2}{\sqrt{4{{x}^{2}}+3}+\sqrt{4{{x}^{2}}+5x+1}}\\ & =\underset{x\to -\infty }{\lim}\,\frac{\frac{2}{- x}- 5}{\sqrt{4+\frac{3}{{{x}^{2}}}}+\sqrt{4+\frac{5}{x}+\frac{ 1}{{{x}^{2}}}}} \\ & =\frac{0-5}{\sqrt{4}+\sqrt{4}}=\frac{-5}{4}. \\ \end{align}\).