It's hard to answer this question without more details. It seems from context we are dealing with a function , which you may recognize more in terms of .

Maybe it's something simple, such as this example in which we have a table of x and f(x)

And we're given that f(x) = 10, so what integer can x be? Then we would reasonably choose x = 3, since f(x) appears to be increasing.Code:x f(x) 1 2 2 6 4 15 5 39