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As the [Ag⁺] needed is very small, it is assumed that the volume of the solution is unchanged on addition of AgNO₃ solution. Therefore, the concentrations of CrO₄²⁻, CO₃²⁻ and Cl⁻ ions are assumed to be unchanged on addition of AgNO₃ solution.

2Ag₂CrO₄(s) ⇌ 2Ag⁺(aq) + CrO₄²⁻(aq) …… Ksp = 1.2 × 10⁻¹²
Ksp(Ag₂CrO₄) = [Ag⁺]² [CrO₄²⁻]
When Ag₂CrO₄ starts precipitation: 1.2 × 10⁻¹² = [Ag⁺]² × 0.20
[Ag⁺] needed to start precipitation of Ag₂CrO₄ = √{(1.2 × 10⁻¹²) / 0.20} M = 2.4 × 10⁻⁶ M

2Ag₂CO₃(s) ⇌ 2Ag⁺(aq) + CO₃²⁻(aq) …… Ksp = 8.1 × 10⁻¹²
Ksp(Ag₂CO₃) = [Ag⁺]² [CO₃²⁻]
When Ag₂CO₃ starts precipitation: 8.1 × 10⁻¹² = [Ag⁺]² × 0.10
[Ag⁺] needed to start precipitation of Ag₂CO₃ = √{(8.1 × 10⁻¹²) / 0.10} M = 9.0 × 10⁻⁶ M

AgCl(s) ⇌ Ag⁺(aq) + Cl⁻(aq) …… Ksp = 1.8 × 10⁻¹⁰
Ksp(AgCl) = [Ag⁺] [Cl⁻]
When AgCl starts precipitation: 1.8 × 10⁻¹⁰ = [Ag⁺] × 0.010
[Ag⁺] needed to start precipitation of AgCl = (1.8 × 10⁻¹⁰) / 0.010 M = 1.8 × 10⁻⁸ M

As the precipitation of AgCl needs the smallest [Ag⁺], AgCl is the first compound to precipitate.