From the given, wee can see that the length of segment JM is 40 units. Segment JM is comprised into the parts JH and HM, and from the Part Whole Postulate, we know that JM = JH + HM. But, since we know the measures of JM and HM (40 units and 21 units respectively), we can substitute their values and solve for JH. The equation becomes:
40 = 21 + JH
We want to figure out what JH is equal to, so we should get all of the constants (terms without a variable) on one side. We can do this by subtracting 21 from each side.
40 - 21 = 21 - 21 + JH
JH = 19
That means that our answer is C, JH = 19 units.