Answer: 
The area of the dam = 175,000 cm^2 
Step-by-step explanation: 
We know that the ratio of the map to the real area is 
![\frac{\text{map}}{\operatorname{Re}al}=(1)/(50000)]()
Now we want the real area of the dam if its area on the map is 3.5 cm^2. 
Let us call x the real area of the dam, then we have 

cross multipication gives 

![\boxed{x=175,000\operatorname{cm}^2}]()
which is our answer!