I'll write "$\displaystyle Z$" to denote the set of whole numbers

Let $\displaystyle n > 1$ be a whole number and let $\displaystyle a$ be a fixed whole number. Prove that

H = {x ∈ Z, such that ax ≡ 0 (mod n)}

is a subgroup of $\displaystyle Z$ in the sum.

In the sum is the part where I'm confused... I though you could only multiply with modules...?