We can use any solution containing chloride ions as NaCl solution or even dilute HCl due to mercurous Hg₂²⁺ is from group I cations which precipitated as chloride while Sr²⁺ is from group V
Net ionic equation:
Hg₂²⁺ + 2 Cl⁻ → Hg₂Cl₂ (white precipitate)
while SrCl₂ is soluble salt