Let (just for visual and latex-ing purposes):

............... (Got rid of the denominator)

...........(Distribute x)

............. (Collect terms containing )

....................... (Factor out)

Divide both sides by (3 - 7x) to isolate . Now you can apply the logarithmic function to both sides to isolate y.

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This is the general format of these questions: Isolate the term containing your variable in the exponent, take the logarithm of both sides and voila.