Решение:

1. a) $$7\sqrt{x} + 2\sqrt{x} - 5\sqrt{x} = (7 + 2 - 5)\sqrt{x} = 4\sqrt{x}$$
2. б) $$2\sqrt{3}(3 - 4\sqrt{75}) - 3\sqrt{12} = 6\sqrt{3} - 8\sqrt{225} - 3\sqrt{4 \times 3} = 6\sqrt{3} - 8 \times 5\sqrt{3} - 3 \times 2\sqrt{3} = 6\sqrt{3} - 40\sqrt{3} - 6\sqrt{3} = (6 - 40 - 6)\sqrt{3} = -40\sqrt{3}$$
3. в) $$3\sqrt{12b} + 0,5\sqrt{108k} - 2\sqrt{48b} + 0,01\sqrt{300k} = 3\sqrt{4 \times 3b} + 0,5\sqrt{36 \times 3k} - 2\sqrt{16 \times 3b} + 0,01\sqrt{100 \times 3k} = 3 \times 2\sqrt{3b} + 0,5 \times 6\sqrt{3k} - 2 \times 4\sqrt{3b} + 0,01 \times 10\sqrt{3k} = 6\sqrt{3b} + 3\sqrt{3k} - 8\sqrt{3b} + 0,1\sqrt{3k} = (6 - 8)\sqrt{3b} + (3 + 0,1)\sqrt{3k} = -2\sqrt{3b} + 3,1\sqrt{3k}$$