The a is a constant that tells us something about the clarity of the water.

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For example:

With a=1 you would have , impossibly clear water.

With a=1/2 you have or , for each metre you descend the light intensity is halved.

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In your question you are told that when x=6m the light intensity is half the surface intensity.

Sub it in your equation .

Can you solve that to find the value of a?