what is the coefficient of x^2 in the power series expansion of (4 + 7x)^(1/3)?
Try a Taylor expansion:
$\displaystyle f(x)=f(a)+(x-a)\frac{\partial}{\partial x} f(a) + \frac{(x-a)^{2}}{2!} \frac{\partial^{2}}{\partial x^{2}} f(a) + ...$
Use $\displaystyle a = 0$, and take the necessary derivatives. The third term in the above series is what you're looking for.