You must look at the slopes to determine whether or not it is a linear function. A linear funtion has a constant slope. So, the slope must be the same throughout the entire function. You would need to use the formula m=(y2-y1)/(x2-x1) to find the slope beween different points that were given. Basically, any inconsistency indicates that it is not a linear function; those coordinates cannot be classified as being a linear function due to the fact that the slope is not constant throughout the function.
