[tex]\sf \sqrt{ {21}^{3} } = \sqrt{4 \times 10 \times 8} \\ \\\sf (21 ^{3} ) ^{ \frac{1}{2} } \sqrt{4 \times 10 \times 8 } \\ \\ (21 ^{3} ) ^{ \frac{1}{2} } \sqrt{320} \\ \\\sf 21 ^{ \frac{3}{2} } \times 8 \sqrt{5} \\ \\ 21 \sqrt{21} \times 8 \sqrt{5} \\ \\\sf 21 \times 8 \sqrt{21 \times 5} = \red{168 \sqrt{105}} [/tex]
