I have a problem about rotation operator in QM.

I must verify that [tex] (\hat{U}(R)f)(\textbf{x})=f(R^{-1}\textbf{x}) [/tex]

with: [tex] \hat{U}(R) = exp({\frac{-i\varphi\textbf{nL}}{\hbar}}) [/tex]

R rotation on versor n of angle [tex]\varphi[/tex]

I don't really know how to start, so please give me an advice!!