Differentiate the function.
f(x) = 7xln(3x) - 7x
a) f'(x) = 7ln(3x) + 7, f''(x) = 7/x
b) f'(x) = 7ln(3x) - 7, f''(x) = 7/x
c) f'(x) = 7ln(3x) + 7, f''(x) = -7/x²
d) f'(x) = 7ln(3x) - 7, f''(x) = -7/x²