Nova Physics states that there is no relationship between spring constant, k, and amplitude. (Ch. 11 question 17 answer) Is this true? I thought that F=kx relates the two, but maybe I'm wrong.
F=kx relates k and x to F, it does not relates k or x to each other.
k and x are independent variables, F is the dependent variable in that case.
The amplitude is determined on how far the spring is displaced, which is arbitrary. You can stretch a slinky and number of lengths, though it always has the same k value.