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Okay, how you do this is using the following equation:

(wavelength)(frequency in Hz) = (speed of light)

Wavelength = speed of light/frequency in Hz

We know that the speed of light is around 3.00 x 10^8

The speed of light is a constant.

Wavelength = (3.00 x 10^8 )/(7.5 x 10^14)
You can do this by doing 3.00/7.5 x 10^8/10^14

When you divide the exponents, you subtract. The exponents are 8 and 14. Since 8 is on top, you subtract it from 14. 8 - 14 = -6
6
3.00/7.5 x 10^-6
Using the correct amount of significant figures, 3.00/7.5 = 0.40

If you don't know about significant figures, make a thread.

So, it is, 0.40 x 10^-6

Now, 0.40 x 10^-6 is not in proper scientific notation.

4.0 x 10^-7 metres is your answer, because for speed of light, I used metres per second.

* Also, you can just plug (3.00 x 10^8 )/(7.5 x 10^14) into a calculator

I always found it easier to remember these sorts of forumlae if I got an understanding of why they worked.

For example, the formula:

wavelength * frequency = speed of wave

Imagine a slow wave, say in a piece of rope.

The wave length is the distance from one peak to another.
The frequence is the number of waves per second.
And the speed of the wave is rthe speed at which one of the peaks moves.

Now try to imagine how these are related. Take an easy example.

A wavelength of one metre, and there is one wave per second, so a frequency of one hertz.

From when we begin until one second has passed we generate a single wave, which is one metre long. So that wave peak has traveled one meter in one second. Otherwise known as a speed of one meter per second.

Back to the formula.

wavelength * frequency = speed of wave

1 * 1 = 1

No brainer really.
Let's try another.

A wavelength of one metre, and there are two waves per second, so a frequency of two hertz.

From when we begin until one second has passed we generate two waves, which each one metre long. So that first wave peak has traveled two metres in one second. Otherwise known as a speed of two metres per second.

1 * 2 = 2

Hopefully that will give you an idea of how the numbers relate to one another.

plopperscioly already covered your specific question, so I will refrain from repeating it.