(B)
if <em>f</em>(a) = K, then that means <em>f </em>sends (a) to (K).
the inverse function, <em>f</em>-1, goes back where you started, so <em>f</em>-1 <em />sends (K) to (a).
<em>f</em>-1(K) = a.
The value of "a" can be anything you want it to be. Usually, it is chosen to make the function meet some additional constraint, such as going through a particular point. You have not provided any such constraint, so there is no rationale for choosing any particular value of "a".
Let a=2, if you like.
a. ((x2-x1)^2 + (y2-y1)^2)^(1/2)
(-4-8)^2 + (1-5)^2 144+16= 160
160^(1/2)= 12.64
b. (8-4)/2 & (5+1)/2
4/2=2 6/2=3
(2,3) is the midpoint of AB