Answer:
Any multiple of 71 (i.e. 71, 142, 710, etc)
Step-by-step explanation:
Republicans = R
Democrats = D
Republicans that liked apple pie = 0.80R
Democrats liked eclairs = 0.55D
Republicans who liked apple pie that also liked eclairs = (0.80*0.20)R
If the number of Republicans who liked both desserts is equal to the number of Democrats who liked eclairs, then the following must be true:

Since the number of Republicans and Democrats must be integers, the least number of members possible is 71 (55+16).
Any value that is a multiple of 71 is a possible value for the number of members of the group
Answer:
I think this is what you want:
2(6)-2x=3
We then see that 2(6) and 2(x) have two in common as a "coefficient" you could say, so we distribute and put parentheses around it.
2(6-x)=3
6-x=3/2
-x=-9/2
x=9/2
Traditional 1/2 * base * height
Heron’s formula (it’s quite lengthy so search it up)
Hope this helped you :)