**You can use this formula, or simply, you can find the inverse function itself and evaluate its derivative when x=3 ..**

**We have \(\displaystyle f(x)=2x^2\) with \(\displaystyle x\geq 0\), so its a one-to-one function since its an increasing function ..**

**Clearly, \(\displaystyle f^{-1}(x)=\sqrt{\frac{x}{2}}=\frac{1}{\sqrt{2}} \cdot \sqrt{x}\) ..**

**Differentiate it, and evaluate its derivative at x=3 ..**