4.74 × 10¹³ Hz.
Explanation
[tex]\text{Frequency} = \dfrac{1}{\text{Time Interval}}[/tex].
In other words,
[tex]f = \dfrac{1}{T}[/tex].
[tex]T = 2.11 \times 10^{-14}\; \text{s}[/tex]
[tex]f = \dfrac{1}{2.11 \times 10^{-14}\; \text{s}}\\\phantom{f} = 4.74 \times 10^{13} \; \text{s}^{-1}[/tex].
Note the unit. [tex]\text{s}^{-1}[/tex] or [tex]\dfrac{1}{s}[/tex] indicates 1 cycle per second. That is the same as 1 Hertz or [tex]\text{Hz}[/tex]. As a result,
[tex]f =4.74 \times 10^{13} \; \text{Hz}[/tex].
