akar 7 (akar 63 - 3 akar 28 =

[tex] \sqrt{7} ( \sqrt{63 } - 3 \sqrt{28} )\\ = \sqrt{7} ( \sqrt{7 \times 9} - 3 \sqrt{7 \times 4}) \\ = \sqrt{7} (3 \sqrt{7} - 3 \times 2 \sqrt{7}) \\ = \sqrt{7} (3 \sqrt{7} - 6 \sqrt{7} )\\ = \sqrt{7} ( - 3 \sqrt{7} ) \\ = - 3 \times 7 \\ = - 21[/tex]