Introduction to systems of linear equations

A point on a plane can be described as a pair of two numbers $(x,y)$ (its coordinates). Then all the solutions of the linear equation $ax + by + c = 0$ describe some set of points on this plane. It turns out that this set is a straight line (that is another meaning of the word ‘linear’). One of the amazing facts about straight lines on a plane is that through any two points you can draw only one straight line. Knowing this, tell which of the following equations describe a line which goes through points $(1,1)$ and $(2,-1)$.