3x + 1 4x - 2
|----------------------------|----------------------------|
r m s
since "m" is the midpoint, thus "m" cuts "rs" in two equal halves, therefore, rm = ms, and thus 3x + 1 = 4x - 2, and surely you'd know what "x" is then.
so rs is just (3x+1) + (4x-2), or just the halves added up.