Explanation:
natural numbers = 1, 2, 3, 4, 5, ...
it is a debate to include also 0. but surely no negative numbers.
-2x <= 9
2x >= -9
x >= -9/2 = -4.5
so, valid x values are all natural numbers.
with or without 0 depends on your teacher.
but normally that would mean
x >= 1, x ∈ N
remember that when multiplying an inequality by a negative number (like -1 in our case) the direction of the inequality flips.