I = -425x^2 + 45500x - 650000
For I to be maximum, dI/dx = 0
dI/dx = -850x + 45500 = 0
x = 45500/850 = 53.53
Therefore, maximum I = -425(53.53)^2 + 45500(53.53) - 650000 = 567,794.11
Therefore, the maximum average annual income, in dollars, a lawyer can earn is $567,794