Look at the graph. On a coordinate plane, a graph increases through (negative 1, 4), levels off at (0, negative 3), and then increases up through (2, 5).

Leslie analyzed the graph to determine if the function it represents is linear or non-linear. First she found three points on the graph to be (–1, –4), (0, -3), and (2, 5). Next, she determined the rate of change between the points (–1, –4) and (0, -3) to be StartFraction negative 3 minus (negative 4) Over 0 minus (negative 1) EndFraction = StartFraction 1 Over 1 EndFraction = 1. and the rate of change between the points (0, -3) and (2, 5) to be StartFraction 5 minus (negative 3) Over 2 minus 0 EndFraction = StartFraction 8 Over 2 EndFraction = 4. Finally, she concluded that since the rate of change is not constant, the function must be linear. Why is Leslie wrong?
The points (–1, –4), (0, –3), and (2, 5) are not all on the graph.
The expressions StartFraction negative 3 minus (negative 4) Over 0 minus (negative 1) EndFraction and StartFraction negative 3 minus (negative 5) Over 2 minus 0 EndFraction both equal 1.
She miscalculated the rates of change.
Her conclusion is wrong. If the rate of change is not constant, then the function cannot be linear.