The word function is derived from a Latin word meaning operation.
Thus, by a given function f(x), a set of mathematical operation/s is meant. It should not be deciphered as a mere multiplication of f and x.
Given function in the problem here, is f(x)=3x-5.
That means, in order to get f(data) we have to do the operation: (3*data - 5).
Thus f(5) = 3*5 - 5 = 15 - 5 = 10.
Therefore, the solution of f(x) at x =5 yields f(5) = 10.