A function is a rule of association between inputs and outputs. When you write
[tex] f(x) = 18x+14 [/tex]
you mean "for every input x, the output is 18 times the input, plus 14".
So, if we know how to act in general, we also know how to act in paticular. If the input is 5, the ouput will be 18 times 5, plus 14. In formula:
[tex] f(5) = 18\cdot 5 + 14 = 90 + 14 = 104 [/tex]
