Can you use the ruler to measure distances? If so, then:

it means unmarked ruler as the only possible action is ''with the help of a ruler you can draw straight lines through two known points''

so far I have: Draw intersection of AC and BD at E, and the intersection of AB and CD to be F. Then, draw EF. E is the centroid of FAD. You can then also construct the medial triangle. However, actually drawing the desired triangle area triangle is more difficult than this.