Answer:
             1.4 × 10¹⁹ molecules of CS₂
Explanation:
The chemical equation is as follow,
                            CS₂  +  3 O₂    →    CO₂  +  2 SO₂
According to this equation,
1.8 × 10²⁴ molecules (3 mole) of O₂ reacts with  =  6.022 × 10²³ molecules (1 mole) of CS₂
So,
         4.21 × 10¹⁹ molecules of O₂ will react with  =  X molecules of CS₂
Solving for X,
                      X  =  (4.21 × 10¹⁹ × 6.022 × 10²³) ÷ 1.8 × 10²⁴
                      X  =  1.4 × 10¹⁹ molecules of CS₂