sqrt(5-sqrt(4-x))=0 how do we do this?

√ [ 5  - √ (4 - x) ]   = 0      square both sides

5 - √ (4 - x)  = 0         add  √ (4 - x)   to both sides

5 =  √ (4 - x)      square both sides again

25  = 4 - x      add x to both sies, subtract 25 from both sides

x = -21