5. [12x/ (5xy^4)] - [9y^2/(15x^3y^2)]
6. [6/(x+2)] + 11/ (x-7)
We find the LCD of the two fractions and that is (x+2)(x-7)6. [6/(x+2)] + 11/ (x-7)
So now we build the fraction to had the LCD
$\displaystyle \frac{6}{(x+2)}\frac{(x-7)}{(x-7)}+\frac{11}{(x-7)}\frac{(x+2)}{(x+2)}=\frac{6x-42}{ (x-7)(x+2) }+\frac{11x+22}{(x-7)(x+2)}$
$\displaystyle =\frac{17x-20}{(x-7)(x+2)}$