Step-by-step explanation:
[tex]3 \sqrt{343} + 3 \sqrt{4} - 3 \sqrt{8} \\ = 3 \sqrt{ {7}^{2} \times 7 } + 3 \sqrt{ {2}^{2} } - 3 \sqrt{ {2}^{2} \times 2 } \\ = 3 \times 7 \sqrt{7} + 3 \times 2 - 3 \times 2 \sqrt{2} \\ = 21 \sqrt{7} + 6 - 6 \sqrt{2} \\ [/tex]
