if i get this right.. to make at least $6,800,000 she should build at least 18 houses, but not more than 38.(d) The developer needs to make at least $6,800,000 (i.e. $6800 thousand) to invest in a block of land she wishes to purchase for her next development project. Find algebraically the roots of the equation P(n) = 6800, and use your answer to suggest how many houses she needs to build to gain this much profit.
560n-10n^2 -6800 = 0
n = 17.80196097 or 38.19803903
She should build 38 houses to make at least $6,800,000
17 < houses built < 39 => at least $6,800,000
other answers are correct.