What is the probability of observing a sample mean of 18 or more from a sample size of 35?

Without knowing the data which is being sampled, it is impossible to answer this other than by saying that the probability is between 0 and 1 inclusive.

Consider a company. If you sample the annual pay of the employees, any mean will be greater than 18 as everyone will be taking home more than £18 per year, so the probability is 1.

Consider a school. If you sample the lengths of feet of the pupils, any mean will be less than 18 as all the feet are less than 18 inches long, so the probability is 0.