Find the value of c such that the three points (5,5), (3,1) and (6,c) lie on the same line.
Note: Three points are on the same line if the slope of the line through any two points is always the same.
To get from 5 to 3 you subtract 2.
To get from 5 to 1 you subtract 4.
So what you do to the x value you double that to the y value.
To get from 5 to six you add 1.
1*2 is 2, 2+5=7