Calculating the section property $Q$ is usually the most confusing part of the the transverse shear stress calculation.

To calculate $Q$, begin by slicing through the cross section at the point of interest in a direction perpendicular to the direction of the internal shear force.

For this problem, the shear stress is required at $D$, located $4 in.$ above the bottom of the tee shape. Since the shear force acts in a vertical direction, we will slice through the tee in a horizontal direction at $D$.