First, determine if DB is vertical in either view. If, for example, it is vertical in the side view, then the rule applies that it is in TL in the front view.
HOWEVER, given that BD is not vertical in either the side or the front view, it means that its TL needs to be determined by some simple descriptive geometry. Your teacher would have taught you how to find the TL of a line, right?
There are a few methods. One method would be to rotate DB in, say, the front view. Do this by placing the compass point on B, open up compass to point D (in the same view) and draw an arc so that D is directly vertically above B (in the front view). Call this new point D2 .
Draw a horizontal line from D2 to where it lies directly above D in the side view. Draw a (very short) vertical line from D (in the side view) until it hits that horizontal line from D2. That point of intersection is the new point D. We'll call it D3
Measure B to D3 (in the side view). That's your TL
Sorry, I don't have the instruments.
