z = 0.11x
Let z represent the mass of hydrogen in a given mass of water (x). If you did parts (a) and (b) of this problem, you know that y = 0.89x.
Parts (c) and (d) assume that x = y + z.
(c) See the attached for a table. z = x - y.
(d) Using the above relationship between x, y, and z, we have ...
... z = x - y = x - 0.89x
... z = 0.11x