Find the first-order derivative of the following functions:
a) f(x) = 5x³ / (4x + 3)
b) f(x) = 3x(4x - 5)³
c) f'(x) = (15x²(4x+3) - 5x³(4)) / (4x+3)²
d) f'(x) = (15x²(4x+3) + 5x³(4)) / (4x+3)²