Answer
$$\approx46.19mi/h$$
Work Step by Step
According to the problem, we have:
$D=ks^2$
$D$ is stopping distance
$s$ is speed
$k$ is constant of proportionality
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$s=40mi/h$
$D=150ft$
$k$ is unknown
$150=k\times 40^2$
$1600k=150$
$k=0.09375 ft\times mi^2/h^2$
So, we have a general formula $D=0.09375\times s^2$
If $D=200ft$
$200=0.09375\times s^2$
$s^2=2133.(3)$
$s\approx46.19mi/h$