We are given with concentration of H⁺, Starting by convertiong this concentration into pH as follow,
pH = - log [H⁺]
pH = - log [0.0035] ∴ conc. of HCl
pH = 2.45
Also,
pH + pOH = 14
pOH = 14 - 2.45
pOH = 11.55
Changing pOH into [OH⁻]
[OH⁻] = 10⁻¹¹°⁵⁵ ∴ Taking Antilog
[OH⁻] = 2.81 × 10⁻¹²