An electromagnetic wave with an electric field that oscillates with a period of 8.43 ps is travelling in a vacuum. what is the wavelength of this electromagnetic wave?

"ps" stands for "pico-second".  It means 10⁻¹² second.

The period of this wave is 8.43 x 10⁻¹² second.

In a vacuum, the speed of any electromagnetic wave is 3 x 10⁸ m/s.

Wavelength = (speed) / (frequency)

But Frequency = 1 / period

So Wavelength = (speed) x (period)

Wavelength = (3 x 10⁸ m/s) x (8.43 x 10⁻¹² sec)

Wavelength = 2.5 x 10⁻³ meter

Wavelength = 2.5 millimeters