Answer:
f(x) = 4 - x
Step-by-step explanation:
'f(x) = blah-blah' is the same as saying 'y = blah-blah', except it is also telling you that the x-y relationship is a function.
I notice that for each case, x + y = 4, so y = 4 - x
Or in 'function' terminology, f(x) = 4 - x