Answer
$14400 \text{ miles per squared hour}$
Work Step by Step
The acceleration $\text{a}$ is the instantaneous rate of change of the speed in a fixed small period of time:
$$a=\frac{d v}{d t} \to dv=a dt \to \int dv=\int a dt \to \int dv=a\int dt $$
$$\to \Delta v=a \Delta t \to a=\frac{\Delta v}{\Delta t} $$
To increase, the variation in speed $ \Delta v$ should be positive.
Notice that $5$ seconds is equivalent to $\frac{5}{3600}$ hours.
$$a=\frac{\Delta v}{\Delta t} \to a=\frac{50-30}{\frac{5}{3600}}=14400 \text{ miles per squared hour} $$
