Answer
$$\left( {\frac{{3\sqrt 2 }}{2},\frac{{3\sqrt 2 }}{2}} \right)$$
Work Step by Step
$$\eqalign{
& \left( {3,\frac{\pi }{4}} \right) \Rightarrow r = 3{\text{ and }}\theta = \frac{\pi }{4} \cr
& {\text{Converting to cartesian coordinates }}\left( {x,y} \right).{\text{ Where}} \cr
& x = r\cos \theta {\text{ and }}y = r\sin \theta \cr
& {\text{We obtain}}{\text{,}} \cr
& x = 3\cos \left( {\frac{\pi }{4}} \right) = 3\left( {\frac{{\sqrt 2 }}{2}} \right) = \frac{{3\sqrt 2 }}{2} \cr
& y = r\sin \left( {\frac{\pi }{4}} \right) = 3\left( {\frac{{\sqrt 2 }}{2}} \right) = \frac{{3\sqrt 2 }}{2} \cr
& {\text{The cartesian coordinates are:}} \cr
& \left( {x,y} \right) = \left( {\frac{{3\sqrt 2 }}{2},\frac{{3\sqrt 2 }}{2}} \right) \cr} $$
