 Solution

The cardioid has nonnegative polar radius $r=1-\cos \theta$, ranging from zero at the right-hand cusp to two on the negative $x$ axis. Its intersections with the circle $r=3/2$ satisfy $\cos \theta =-1/2$, giving $\theta =2\pi /3,4\pi /3$. The selected region is the left-hand cap between the two curves:The endpoints in Cartesian coordinates are $(-3/4,\pm 3\sqrt{3}/4)$.