The relation “congruence modulo m” is

If R be the relation ,xRy ⇔x-y is divisible by m ,xRx because x-x is divisible by mSo R is reflexive.xRy ⇒yRx so R is symmetric xRy and y Rz ⇒x-y=k1m    y-z=k2mx-z =(k1+k2)m so R is transitive As R is reflexive,symmetric and transitive.it is an equivalence relation