For x belonging to Z, let us denote by [x] equivalence class which contains x. Prove that if [x]=[x'] and [y]=[y'] then [x+y]=[x'+y'] and [xy]=[x'y']

Remark: When xRy people say that "x and y are congruent mod 3", and sometimes write x equivalent y (mod3)

someone can help me please here???