slope (gradient) : (y2 - y1) / (x2 - x1)
(2,1)..x1 = 2 and y1 = 1
(5,k)..x2 = 5 and y2 = k
slope : (k - 1) / (5 - 2) = 1/6
(k - 1) / 3 = 1/6
k - 1 = 1/6 * 3
k - 1 = 1/2
k = 1/2 + 1
k = 1/2 + 2/2
k = 3/2 or 1.5
check..
(3/2 - 1) / (5 - 2) = 1/6
(3/2 - 2/2) / 3 = 1/6
(1/2) / 3 = 1/6
1/2 = 1/6 * 3
1/2 = 3/6
1/2 = 1/2 (correct)
k = 3/2 or 1.5
