A relationship of direct proportionality that, when plotted on a graph, traces a straight line. In linear relationships, any given change in an independent variable will always produce a corresponding change in the dependent variable.

Thus, d = at + k

14 = 0⋅a + k

20 = 1⋅a + k

⇒

k = 14

a = 6

⇒

d = 6t + 14