self doubt

Question

given two dcfl g1 and g2 is L(g1) INTERSECTION L(g2)=Φ is decidable?

the solution is like 

but doubt is that how they used demorgens law....

 

Comments

The De morgan law's application is wrongly used :/

Ref1 : https://gateoverflow.in/224870/decidable-undecidable-problem-decidable-undecidable-problems?show=227000

Ref2:https://gateoverflow.in/29618/is-the-intersection-of-a-cfl-with-another-cfl-decidable

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THANK U SO MUCH