Решение:

а) $$rac{3x-1}{x^2} - rac{x-9}{3x} = rac{3(3x-1) - x(x-9)}{3x^2} = rac{9x-3 - x^2 + 9x}{3x^2} = rac{-x^2 + 18x - 3}{3x^2}$$

б) $$rac{1}{2a-b} - rac{1}{2a+b} = rac{(2a+b) - (2a-b)}{(2a-b)(2a+b)} = rac{2a+b - 2a + b}{4a^2 - b^2} = rac{2b}{4a^2 - b^2}$$

в) $$rac{5}{c+3} - rac{5c-2}{c^2+3c} = rac{5}{c+3} - rac{5c-2}{c(c+3)} = rac{5c - (5c-2)}{c(c+3)} = rac{5c - 5c + 2}{c(c+3)} = rac{2}{c(c+3)} = rac{2}{c^2+3c}$$