-- Mathlib/CategoryTheory/Monoidal/Bimod.lean
theorem pentagon_bimod {V W X Y Z : Mon_ C} (M : Bimod V W) (N : Bimod W X) (P : Bimod X Y)
    (Q : Bimod Y Z) :
    whiskerRight (associatorBimod M N P).hom Q ≫
      (associatorBimod M (N.tensorBimod P) Q).hom ≫
        whiskerLeft M (associatorBimod N P Q).hom =
      (associatorBimod (M.tensorBimod N) P Q).hom ≫
        (associatorBimod M N (P.tensorBimod Q)).hom

Proof state:
case h.h
C : Type u₁
inst✝⁴ : Category.{v₁, u₁} C
inst✝³ : MonoidalCategory C
inst✝² : HasCoequalizers C
inst✝¹ : ∀ (X : C), PreservesColimitsOfSize.{0, 0, v₁, v₁, u₁, u₁} (tensorLeft X)
inst✝ : ∀ (X : C), PreservesColimitsOfSize.{0, 0, v₁, v₁, u₁, u₁} (tensorRight X)
V W X Y Z : Mon_ C
M : Bimod V W
N : Bimod W X
P : Bimod X Y
Q : Bimod Y Z
⊢ ((coequalizer.desc (AssociatorBimod.homAux M N P) ⋯ ▷ Q.X ≫
          colimit.ι
            (parallelPair (TensorBimod.actRight M (N.tensorBimod P) ▷ Q.X)
              ((α_ (TensorBimod.X M (N.tensorBimod P)) Y.X Q.X).hom ≫ TensorBimod.X M (N.tensorBimod P) ◁ Q.actLeft))
            WalkingParallelPair.one) ≫
        coequalizer.desc (AssociatorBimod.homAux M (N.tensorBimod P) Q) ⋯) ≫
      colimMap
        (parallelPairHom (M.actRight ▷ TensorBimod.X (N.tensorBimod P) Q)
          ((α_ M.X W.X (TensorBimod.X (N.tensorBimod P) Q)).hom ≫ M.X ◁ TensorBimod.actLeft (N.tensorBimod P) Q)
          (M.actRight ▷ TensorBimod.X N (P.tensorBimod Q))
          ((α_ M.X W.X (TensorBimod.X N (P.tensorBimod Q))).hom ≫ M.X ◁ TensorBimod.actLeft N (P.tensorBimod Q))
          ((M.X ⊗ W.X) ◁ coequalizer.desc (AssociatorBimod.homAux N P Q) ⋯)
          (M.X ◁ coequalizer.desc (AssociatorBimod.homAux N P Q) ⋯) ⋯ ⋯) =
    coequalizer.π (TensorBimod.actRight (M.tensorBimod N) P ▷ Q.X)
        ((α_ (TensorBimod.X (M.tensorBimod N) P) Y.X Q.X).hom ≫ TensorBimod.X (M.tensorBimod N) P ◁ Q.actLeft) ≫
      coequalizer.desc (AssociatorBimod.homAux (M.tensorBimod N) P Q) ⋯ ≫
        coequalizer.desc (AssociatorBimod.homAux M N (P.tensorBimod Q)) ⋯

Tactic:
slice_lhs 2 3 => rw [coequalizer.π_desc]

Result:
case h.h
C : Type u₁
inst✝⁴ : Category.{v₁, u₁} C
inst✝³ : MonoidalCategory C
inst✝² : HasCoequalizers C
inst✝¹ : ∀ (X : C), PreservesColimitsOfSize.{0, 0, v₁, v₁, u₁, u₁} (tensorLeft X)
inst✝ : ∀ (X : C), PreservesColimitsOfSize.{0, 0, v₁, v₁, u₁, u₁} (tensorRight X)
V W X Y Z : Mon_ C
M : Bimod V W
N : Bimod W X
P : Bimod X Y
Q : Bimod Y Z
⊢ coequalizer.desc (AssociatorBimod.homAux M N P) ⋯ ▷ Q.X ≫
      AssociatorBimod.homAux M (N.tensorBimod P) Q ≫
        colimMap
          (parallelPairHom (M.actRight ▷ TensorBimod.X (N.tensorBimod P) Q)
            ((α_ M.X W.X (TensorBimod.X (N.tensorBimod P) Q)).hom ≫ M.X ◁ TensorBimod.actLeft (N.tensorBimod P) Q)
            (M.actRight ▷ TensorBimod.X N (P.tensorBimod Q))
            ((α_ M.X W.X (TensorBimod.X N (P.tensorBimod Q))).hom ≫ M.X ◁ TensorBimod.actLeft N (P.tensorBimod Q))
            ((M.X ⊗ W.X) ◁ coequalizer.desc (AssociatorBimod.homAux N P Q) ⋯)
            (M.X ◁ coequalizer.desc (AssociatorBimod.homAux N P Q) ⋯) ⋯ ⋯) =
    coequalizer.π (TensorBimod.actRight (M.tensorBimod N) P ▷ Q.X)
        ((α_ (TensorBimod.X (M.tensorBimod N) P) Y.X Q.X).hom ≫ TensorBimod.X (M.tensorBimod N) P ◁ Q.actLeft) ≫
      coequalizer.desc (AssociatorBimod.homAux (M.tensorBimod N) P Q) ⋯ ≫
        coequalizer.desc (AssociatorBimod.homAux M N (P.tensorBimod Q)) ⋯