I don't remember what is a extraneous solution, but i hope it helps
a)
his income must be "x" which is the 100%, and we would know that 0.2% of that is 12.37.
b)
his income must be "x" which is the 100%, and we would know that 2% of that is 12.37.
btw he's not making much either way.
Slope: 3/2
Y-Intercept: -3