if we do a vertical line test on that graph, we'll find that a vertical line will pass by and hit the line only once on its way down, only once meaning the graph is the graph of a function.
Range
well, is really how high and low it goes or namely over the y-axis.
well, it goes up up up to 3 then U-turns and back down it goes, now the graph has arrowheads, meaning the graph keeps on going towards infinity, vertically as well as horizontally since it's a parabola, so the range will be
[3 , +∞)
Domain
well, is simply how left and right it goes or namely over the x-axis.
judging from the arrowheads it moves from infinity to infinity, so
(-∞ , +∞).