If it is a linear function, then the same difference in x produces the same difference in y. Top left: The difference in y is always 2 even though hte difference in x is not. Not a linear function. Bottom left: Difference in x is always 1, but difference in y is 2 or 3. Not a linear function. Top right: Difference in x is always 2. Difference in y is always 2. This is a linear function. Bottom right: Difference in x is always 2, but difference in y is 2 or 3. Not a linear function.
