rw [mul_assoc]
s a b : ℝ
⊢ (sqrt π)⁻¹ * s * ∫ (x : ℝ) in a..b, rexp (-(s * x) ^ 2) = (sqrt π)⁻¹ * ∫ (x : ℝ) in s * a..s * b, rexp (-x ^ 2)
s a b : ℝ
⊢ (sqrt π)⁻¹ * (s * ∫ (x : ℝ) in a..b, rexp (-(s * x) ^ 2)) = (sqrt π)⁻¹ * ∫ (x : ℝ) in s * a..s * b, rexp (-x ^ 2)