equation form: y = mx + b
where m is the slope, and b is the y-intercept.
slope = (y2 - y1) / (x2 - x1) = (13 -8) / (11 - 5) = 5/6
y = 5/8 x + b (replace x and y by 5 and 8)
=> 8 = 5/6 * 5 + b
=> 8 = 25/6 + b
=> b = 8 - 25/6 = 23/6
==> equation: y = 5/6 * x + 23/6
