There are some missing terms. First of all, I assume that A, B, and C are collinear and that B is between A and C.
If this is true then AC-AB=BC by the whole is the sum of its parts theorem.
24-20=4
Otherwise, all that can be said about BC is that it's length is between AC-AB = 4 and AC+AB = 44 units.
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10
6.7
C minus B equals AB
associative property
2