Answer
$0.39$ Hz
Work Step by Step
We know that the formula for the period is:
$$T=2\pi \sqrt{\frac{l}{g}}$$
We also know that frequency is:
$$f=\frac{1}{T}$$
The two formulas above can be combined as:
$$f=\frac{1}{2\pi} \sqrt{\frac{g}{l}}$$
Since the elevator is accelerating at $2.0 m/s^2$ upward, the gravity in the elevator is $9.8m/s^2+2.0m/s^2=11.8m/s^2$. Therefore, the frequency is: $$f=\frac{1}{2\pi} \sqrt{\frac{11.8m/s^2}{2.0m}}=0.39 Hz$$
