в) $$ \frac{15x-15y}{(5x-3y)(5x+3y)} + \frac{4}{-(3y+5x)} = \frac{15(x-y)}{(5x-3y)(5x+3y)} - \frac{4}{5x+3y} = \frac{15(x-y) - 4(5x-3y)}{(5x-3y)(5x+3y)} = \frac{15x - 15y - 20x + 12y}{(5x-3y)(5x+3y)} = \frac{-5x - 3y}{(5x-3y)(5x+3y)} = \frac{-(5x+3y)}{(5x-3y)(5x+3y)} = \frac{-1}{5x-3y} $$
Ответ: $$ \frac{-1}{5x-3y} $$