Answer:
f(x) = 0.42x ; x is the input as the length of rope in feet
; f(x) is the output as cost
Explanation:
For a relation to be a function there is a condition that in a function every input has a unique output. It means that for one input there should be only one output, two or more outputs are not allowed.
The function f(x)=0.42x satisfies the above condition to become a function.
(To find the function use the following method
Cost of total length of rope = (Cost of rope per foot) × (Length of rope in feet)
f(x) = 0.42 × x
f(x) = 0.42x
)