The function could be expressed using the absolute value symbol if necessary or as a constant function f(x) = k. It could also take the form of a piecewise-defined function or be plotted using a TI-83/84 calculator if variance in x requires it.
For the function f(x) given to be defined for 0 ≤ x ≤ 20, and since the graph of f(x) is a horizontal line, we can express this function in two different ways. One possible expression is using the absolute value symbol which would usually be like |x-a|, but since it's a constant function, the absolute value is not needed.
The simpler expression is just the constant value, for instance, f(x) = k, where k is a constant. However, without the actual function provided in the question, we cannot determine the correct expression for f(x).
Moreover, if the function's definition varies over different intervals of x, it could be expressed as a piecewise-defined function where different expressions describe f(x) depending on x's value range.
Alternatively, if you're using technology such as a TI-83, 83+, or 84 calculator, you could use its function capabilities to input and plot the function directly, allowing you to visualize the piecewise nature or constant value if applicable.