Depends on what language you're using. In math notation, given `y = x * x`, you can work backwards from `y = 4` to figure out the value of x, whereas in, say, Javascript, `y = x * x` means exactly "compute y as the value of x times itself" and only that. For illustration, we could also compute the square of x in a different imperative form, e.g. in terms of a loop over additions.
Similarly, in mathematical notation, `f(g(x))` can be a way of expressing the existence of some sort of law, e.g. maybe f and g are commutative. That means that if code were written as such in a 5th-gen language[1], the underlying engine is free to recompile the code into `g(f(x))` assuming the commutative property holds and the performance is better. By contrast, in a imperative language, `f(g(x))` generally would compile to that exact order of operations (unless you have a mythical sufficiently smart compiler)
I can see an argument about JIT compilers being smart in some cases, but the philosophical distinction between imperative and declarative paradigms is that with declarative style, the compiler can transparently swap units of arbitrary complexity. For example, given some CSS rules, a browser engine can decide to paint the screen buffer however it wants, be it top-to-bottom, edge-to-center, layer-over-layer, etc regardless of how the CSS was originally expressed.
Compiled programming languages are declarative ways of generating machine code. The source code describes what the output or final state (the executable) should consist of, but not how to construct it (that's in the compiler source).
Is the code "read x; print x + 5" declarative or imperative?
It's declarative because it doesn't specify how to read the number, how to print the result, or how to add numbers. It merely symbolically describes the IO and calculations to be performed.
It's imperative because it specifies in a step-wise fashion reading input, performing a calculation, and outputting the result.
Declarative code is imperative from the perspective of the next layer up in the abstraction stack. Declarative code elides implementation details; the we call the implementation details imperative, because they specify the "how" and not the "what", which is the domain of the higher level.
Under this lens, what can we say about this:
arr
.map(x => x + 2)
.filter(x => x % 3)
.map(x => other(x))
It's imperative if we understand map() and filter() to be imperative operations. If they're declarative - perfectly possible in C# - then the code is declarative, because `arr` could be quite abstract, and do something much more interesting.
Declarative code describes the _output_ or final desired state of something.