What is the difference between finding the derivative of y = (3x)^2 and y = 3x^2?

(3x)^2  =9x^2    and the derivative of this = 18x

3x^2   .....and the derivative of this = 6x

derivative of y = (3x)^2

=y'(x) = 18x

derivative of y = 3x^2?

y'(x) = 6x