if (3,21) lies on the graph of f, then f()= what?
There are an infinite number of functions that pass through that point. You must tell more, like if a line give the slope.
To determine the value of f(x) when (3, 21) lies on the graph of f, we need additional information or context. The pair (3, 21) represents a point (x, y) on the graph of f, but without knowing the function f(x) or any other points on the graph, it is impossible to calculate f(x) or determine its value.
If you have the function or any other relevant information about f(x), please provide it, and I'll be happy to help you find the value of f() in that context.