A) 3 houses
B) 5 houses
Frankly, for this question, using advanced mathematics isn't needed since x can only be an integer and the function needs to be evaluated at only 14 points. So let's evaluate the function for the values of x ranging from 0 to 13 and see what we get:
f(0) = -1673
f(1) = -708.848
f(2) = 0.216
f(3) = 485.104
f(4) = 776.728
f(5) = 906
f(6) = 903.832
f(7) = 801.136
f(8) = 628.824
f(9) = 417.808
f(10) = 199
f(11) = 3.312
f(12) = -138.344
f(13) = -195.056
Looking at the above values, the answer to "How many should the builder construct in order to have a profit of at least $400,000?" is rather obvious. That would be 3 houses with a profit of about $485,000. Yes, building 9 houses would get a profit closer to $400,000; But doing three times the effort for less profit isn't a reasonable business choice. And to maximize profit, the obvious choice becomes 5 houses since that's the largest result.