Showing 2 sets have equal cardinality

Question

$$A = \{ 3n+1 \mid n \in \mathbb Z \}$$

$$B = \{ 7m \mid m \in \mathbb Z \}$$

I know I have to find a bijection between them, but do I write out numbers in each set and try to guess a function to get from one set to another?

Answer 1

Hint: There's an obvious bijection from $\mathbb Z$ onto each of them.