9) question doesn't say what it's lowered to but let's say it's lowered from 15% to 10%. you would just change the 15 to a 10.
g(s) = 4000 + 0.10s
10) 4000 is the base salary, doubled = 8000
h(s) = 8000 + 0.15s
11) cut the 4000 in half = 2000
k(s) = 2000 + 0.15s