21. the values of x such f(x) < g(x):

22. The values of x such g(x) < 0:
Let
be the number of liters of the 10% solution to be used, and
the number of liters of the 4% solution. The chemist wants to end up with a 4 liter solution, so

For each liter used in the mixture, a concentration of either 10% or 4% acid will be contributed, and the goal is to make a 4L solution whose concentration is 7%, which means

Solving for
gives
, so 2L of both solutions is needed.
The answer is T=PV/nR
Hope this helps!