Answer:
22+18-6=34
Step-by-step explanation:
Let's call F the set of dancers who dance fox-trot (not necessarily just that, they may also dance something else, what's important is that, amongst other things, they also dance fox-trot), and T the set of dancers who dance tango.
We know that F has 22 elements, T has 18, and the intersection of sets F and T (e. g. those who dance both tango and fox trot) has 6 ones.
Now, let's take a further look at how those sets are made up.
Let's take set F, for example. In it, there is some number of dancers who dance only fox trot, and some others who dance both fox trot and tango.
Similarly, set T is made up of those who dance only tango and those who dance both fix trot and tango.
Therefore, if we were to add those sets together, we would get the number of those who only dance tango, plus the number of those who only dance fox trot, plus two times the number of those who dance both.
We can make up for that, and get the desired result, by subtracting the union of sets F and T from their sum.
Hope this was a clear explanation :)
