The rule of the function is : f(x) = 2x - 3
Using the pair of points from the graph to write the rule of the linear function in the form: y = bx + c
b = slope = (Rise / Run)
b = (1 - 5)/(2 - 4)
b = -4/-2
b = 2
y = 2x + c
Using the point (2, 1)
1 = 2(2) + c
1 = 4 + c
c = -3
Now we have :
Hence, the rule of the function is : f(x) = 2x - 3