The definition of log to the base 7 of a number is the answer to the question "To what power do you have to raise 7 in order to get your number?". To what power do you have to raise 7 to get 73? Answer 3.
I am assuming this =y, even though you did not indicate such.
This resolves by moving the base of 7 from the left side, to the right side where it serves as the base of y as an exponent.
If I have made incorrect assumptions, please let me know.
From one Vivian to another, I should be able to read your mind.