apply Quotient.sound
case h
R : Type
inst✝² : Ring R
inst✝¹ : DecidableEq R
X : Type
inst✝ : DecidableEq X
a : R
x₁ x₂ : R[X]
s₁ s₂ : FormalSum R X
⊢ a • (Quotient.mk (formalSumSetoid R X) s₁ + Quotient.mk (formalSumSetoid R X) s₂) =
    a • Quotient.mk (formalSumSetoid R X) s₁ + a • Quotient.mk (formalSumSetoid R X) s₂
case h.a
R : Type
inst✝² : Ring R
inst✝¹ : DecidableEq R
X : Type
inst✝ : DecidableEq X
a : R
x₁ x₂ : R[X]
s₁ s₂ : FormalSum R X
⊢ scmul a (s₁ ++ s₂) ≈ scmul a s₁ ++ scmul a s₂