Answer: 
Step-by-step explanation:
Given
Propane produced heat energy of 
The volume of the tank is 
The energy produced by this propane tank is

Similar triangles so AC:BC=AE:DE
15+6=21 = AE
15:12.5=21:?
to get from 15 to 12.5 you divide by 15 and times by 12.5 so do this the to the other side
21/15=1.4
1.4 x 12.5=17.5
So DE=17.5
You can tell that a line is parallel if the lines can go on forever without crossing or intersecting. A line is perpendicular when the lines intersect at a right angle. If the slopes are either equal or negative reciprocals, they cannot be parallel or perpendicular.
Hope this helps you!