no goals
R : Type u
inst✝ : CommRing R
I J : Ideal R
⊢ RingHom.ker (quotQuotMk I J) = I ⊔ J
rw [RingHom.ker_eq_comap_bot, quotQuotMk, ← comap_comap, ← RingHom.ker, mk_ker,
  comap_map_of_surjective (Ideal.Quotient.mk I) Quotient.mk_surjective, ← RingHom.ker, mk_ker,
  sup_comm]