Answer:
Decrease by L1L2 in sector 1
Explanation:
Labour markets are at equilibrium when market demand for labour = market supply of labour.
Sector 1 previous equilibrium is 0L1, equilibrium wage is 0Wn (0 is origin). Union leads to increase in wage rate from 0Wn to 0Wu. This increase in wages lead to decrease in employment in sector 1. Supposing that the new equilibrium labour employed = 0L2, which is lesser than previous employment 0L1. So, the employment level falls by margin difference L1L2 in sector 1.