Ionic Equilibrium

At 25 °C, the concentration of H⁺ ions in 1.00 × 10⁻³ M aqueous solution of a weak monobasic acid having acid dissociation constant (K_a) of 4.00 × 10⁻¹¹ is X × 10⁻⁷ M. The value of X is ______.

Use: Ionic product of water (K_w) = 1.00 × 10⁻¹⁴ at 25 °C