Answer:
Professor Johann Bernoulli was a very busy person, and refused to give Euler private lessons.
Explanation:
Johann Bernoulli would not teach Leonhard Euler privately, mainly on superficial math problems.
But Euler received from Johann Bernoulli a valuable advice to read more advanced math books on his own and work on more elaborated problems, and only if he could not be able to solve a equation he was allowed to visit Johan on Sunday afternoon to help him.