Answer
$a=53.777ft/s^2$
Work Step by Step
All we have to use is the slightly uncommon formula for a constant acceleration system. That is, $v_f^2 = v_i^2 +2al.$ Substituting the given values,
$$a=\frac{(50mi/h)^2}{2\cdot 100ft}=\frac{(73.3333ft/s)^2}{100ft}=53.777ft/s$$
