There seems to be some confusion in the question. The expression "5+7-21" does not appear to be related to the rest of the question. Additionally, the function f(x) is defined as a constant function f(x) = 100, which means that there are no gaps in its domain.
Assuming that the intended question is to compute lim f(z) as z goes to some value, we can simply apply the definition of the limit for a constant function:
lim f(z) = f(z) = 100
This means that the limit of f(z) as z approaches any value is equal to 100.