a)
\frac{x+2}{x^2-9}-\frac{x+4}{x+3}=\frac{x+2}{x^2-9}-\frac{(x+2)(x-3)}{x^2-9}=\frac{x+2-(x^2-3x+2x-6)}{x^2-9}=
=\frac{x+2-x^2+x+6}{x^2-9}=\frac{-x^2+2x+8}{x^2-9}
b)
\frac{3x-1}{x+2}+ \frac{x^2-2}{x^2-4}=\frac{(3x-1)(x-2)}{x^2-4}+\frac{x^2-2}{x^2-4}=\frac{3x^2-6x-x+2+x^2-2}{x^2-4}=\frac{4x^2-7x}{x^2-4}
c)
\frac{4x}{x+6}-\frac{x+5}{x^2+6x}=\frac{4x*x}{x^2+6x}-\frac{x+5}{x^2+6x}=\frac{4x^2-x-5}{x^2+6x}