Chain rule by example, in short:
The derivative of 7x^3 is obviously 21x^2. This is the power rule.
What if instead of having "x", you have some other expression.. like, for example, (2x+1)? How would you find the derivative of 7(2x+1)^3 ?
Use the power rule + chain rule. First, do it the way you normally would with the power rule:
21(2x+1)^2
But it can't be that simple, right? Right. You have to take into account what's inside the expression (in this case, 2x+1). Simply take the derivative of that and multiply it by the previous result.
21(2x+1)^2*2
And that's your answer.