This should be in MSM $$\frac{2+5+14+15+x}{5}=x$$so $$2+5+14+x+15=5x$$then $$2+5+14+15=4x$$finally $36=4x$ so $$x=9$$

As $x$ is the average, including it in the average of the five numbers does not affect the average, so $x$ is just the average of the other four numbers, which is $9$.