That's the definition of logarithms.
The problem gives which is the same as

Look at the right side.

Back to the definition of logarithms. Turn it into an equation because now we want to find its value.

means

Remember that .

So the answer is y=-2 because

so substitute -2 into the right side of the equation.

Subtract 7 from both sides, then divide both sides by 3.

Check the original equation.

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