How do you determine the value of f of x given a value of x?

To determine the value of f(x) given a specific value of x, you substitute that x value into the function f(x). This involves performing the necessary arithmetic operations or calculations as defined by the function. For example, if f(x) = 2x + 3 and you want to find f(4), you would calculate f(4) = 2(4) + 3, resulting in f(4) = 11.