How many different Boolean functions F(x,y,z) are there such that
F([x],y,z) = F(x,[y],z) = F(x,y,[z])
for all values of the Boolean variables x, y, and z?
 means the complement of (-)
I don't even understand the question.. I would understand if it asked something like "..F(x,y,z) = xy +z" .. but sadly it does not.
Hint: Write F in full disjunctive normal form, and note conditions such as F(x,y,z)=1 implies F contains
. Similarly ... and hence you only have free choice for
and so there are ... such functions.