Question 924464
Gradient = y2 - y1/x2 - x1
Gradient of the line connecting (1,1) and (2,2)
 =  2 - 1/2 - 1 = 1
Gradient of the line connecting (2,2) and (3,4)
 =  4 - 2/3 - 2 = 2
Different gradients, therefore not on the same straight line.

Hope this helps.
:-)