use the definition at top to solve only 2.11

Definition. Let Zn={[a]naZ}. If the modulus n is given or irrelevant, we write just [a] for [a]n. Example 2.10. Find (with proof) the cardinality of Zn. Example 2.11. Suppose aZ and nN. Prove or disprove that [a]nb= [ab]n, where [a]nb is the equivalence class given by btimes[a]n[a]n[a]n. If b is the residue of bmodn, prove or disprove that [ab]n=[a]nb