You haven't told us whether ABC is a line or a triangle.
-- IF ABC is a line, and if 'B' is on the line between 'A' and 'C',
and if AB=3.7m and AC=8.5m, then . . .
BC = (8.5m - 3.7m) = 4.8meters.
-- But IF ABC is a triangle, and if 'A', 'B', and 'C' are the vertices
of the triangle, and if AB=3.7m and AC=8.5m, then BC can be
ANY length that's more than 4.8 meters and less than 12.2 meters.
