Answer is: 2940 mL of the HCL solution.
c₁(HCl) = 10.0 M.
V₂(AgNO₃) = ?.
c₂(AgNO₃) = 0.85 M.
V₁(AgNO₃) = 250 mL ÷ 1000 mL/L = 0.25 L.
c₁ - original concentration of the solution, before it gets diluted.
c₂ - final concentration of the solution, after dilution.
V₁ - volume to be diluted.
V₂ - final volume after dilution.
c₁ · V₁ = c₂ · V₂.
V₂(HCl) = c₁ · V₁ ÷ c₂.
V₂(HCl) = 10 M · 0.25 L ÷ 0.85 M.
V₂(HCl) = 2.94 L · 1000 mL = 2940 mL.