Here the difference between two consecutive x terms =14-18=-4
And we have to use the following formula
f(x) = a + (x-1)d
Substituting the values of first term, a common deifference,d, we will get
f(x) = 18+(x-1)(-4)
f(x) = 18 -4x+4
f(x) = 22-4x
So the required model for the given table is f(x) = 22-4x .