Answer
Maximum revenue of ${{\$}} 14,400,000$ occurs when
$120$ houses are built.
Work Step by Step
We have data points
$(p, q)=(50,190000)$ and $(70, 170000)$.
The line passing through these points is
$q-q_{1}=\displaystyle \frac{q_{2}-q_{1}}{p_{2}-p_{1}}(p-p_{1})$
$q=\displaystyle \frac{-20,000}{20}(p-50)+190,000$
$q=-1000p+240,000$.
Revenue is $R=pq$
$R(p)=-1000p^{2} +240,000\mathrm{p}$.
The graph of $R(p)$ is a parabola that opens down (leading coefficient is negative).
The vertex is the point of maximum.
$-b/(2a)=-\displaystyle \frac{240,000}{-2\times 1000}=120$ houses
The corresponding revenue is
$R={{\$}} 14,400,000.$
