Answer
$3$
Work Step by Step
Step 1. Assume the logarithmic function has a form of $y=log_bx$
Step 2. As point $(81,4)$ is on the graph, we have $4=log_b81$ or $b^4=81$, thus $b=3$
You can help us out by revising, improving and updating this answer.
Update this answerAfter you claim an answer you’ll have 24 hours to send in a draft. An editor will review the submission and either publish your submission or provide feedback.