Answer
$(4R/3\theta)sin(.5\theta)$ from the tip of the slice.
Work Step by Step
We know that the area is:
$A=\frac{\pi r^2 \theta}{2\pi}=\frac{ r^2 \theta}{2}$
Thus, we find that the center of mass, using the geometric center of the slice of pizza, is:
$y_{cm}=\frac{.67r^3sin.5\theta}{ r^2 (.5\theta)}$
$y_{cm}=(4R/3\theta)sin(.5\theta)$ from the tip of the slice.
