Universal Property of Quotient Ring

Theorem
Let $R,S$ be commutative rings.

Let $I\trianglelefteq R$ be an ideal.

Let $\pi : R\to R/I$ be the projection.

Let $f:R\to S$ be a ring homomorphism with $f(I)=0$.

Then there exists a unique ring homomorphism $\overline f:R/I \to S$ such that $f = \overline f \circ \pi$.

Also see

 * First Isomorphism Theorem for Rings