simp_rw [← mul_assoc, ← coeff_derivative]
case neg.a
R : Type u_1
inst✝ : CommRing R
p : R[X]
r : R
hp : ¬p.natDegree = 0
i : ℕ
⊢ p.coeff (i + 1) * ((↑i + 1) * r ^ (p.natDegree - (i + 1))) =
    p.coeff (i + 1) * ((↑i + 1) * r ^ ((derivative p).natDegree - i + (p.natDegree - ((derivative p).natDegree + 1))))
case neg.a
R : Type u_1
inst✝ : CommRing R
p : R[X]
r : R
hp : ¬p.natDegree = 0
i : ℕ
⊢ (derivative p).coeff i * r ^ (p.natDegree - (i + 1)) =
    (derivative p).coeff i * r ^ ((derivative p).natDegree - i + (p.natDegree - ((derivative p).natDegree + 1)))