Is the domain {1.8,3.4,9,9.8}

{3.2,3.9,5.8,8.4,9.1}

{2.4,4.8,6.3,8.8,10.1}

{3,4.6,6.7,8.9,10.8}

You need to find the values of x that correspond to the given values of f(x). That is, you need to solve for x:
.. y = 7x -2.7
.. y +2.7 = 7x
.. x = (y +2.7)/7

You are given y values of {14.1, 30.9, 41.4, 58.9, 68}. The corresponding x values are {2.4, 4.8, 6.3, 8.8, 10.1}, corresponding to selection C.

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Since the first number in each choice is different, you only need to evaluate f(x) for those numbers to see which gives the correct result. Only the 3rd one (2.4) give f(2.4) = 14.1, the correct answer.

That they are numbers and related to each other.