Let f(x) = x³ + 3x² - 45x + 7. Use the definition of a derivative or the derivative rules to find f'(x) = _____.

a) 3x² + 6x - 45
b) 3x² + 6x + 45
c) 3x² - 6x - 45
d) 3x² - 6x + 45