$$\begin{aligned} 2\sqrt{3} - 2\sqrt{17} \cdot \sqrt{51} &= 2\sqrt{3} - 2\sqrt{17} \cdot \sqrt{3\cdot 17} \\ &= 2\sqrt{3} - 2\sqrt{17} \cdot \sqrt{3} \cdot \sqrt{17} \\ &= 2\sqrt{3} - 2 \cdot 17 \cdot \sqrt{3} \\ &= 2\sqrt{3} - 34\sqrt{3} \\ &= -32\sqrt{3} \end{aligned}$$
Ответ: -32$$\sqrt{3}$$