Answer: No, it is not a linear function
Step-by-step explanation:
We can see that it's a function because it passes the vertical line test, but it's not linear. To be linear, it needs to be one single straight line only (instead of composed of multiple pieces of different straight lines). This graph is considered a piecewise function.
So if your teacher asked "is this a function?" then the answer would be "yes". However, the "linear" portion is what makes the final answer "no".