Observe the graph,

The left portion of the graph is the line .

There is an open circle at , so the domain for this part of the function is .

The center portion of the graph is the constant function .

There are closed dots at and , So the domain for this part is .

The right portion of the graph is the line .

There is an open circle at , So the domain for this part is . Therefore the function is .

The function is .