The answer you are looking for is f(x) = x^2 (x squared).
To find this, you look at a common places within each sequence (x = 1; f(x) = 1, x = 49 f(x) = 7, etc). When f(x) = 1; x = 1 and when f(x) = 7; x = 49. With this information, you figure out what you'd multiply to each f(x) value to get the x value. Thus meaning f(x) = x^2.
