The seventh grade mathematics real number operation seeks the detailed answer and the instruction.
Because the coefficient before the brackets is 2, which means 2 times, so you have to subtract 2 times. Of course it's negative 4! Can be calculated as follows
:2[9+2×(√5-2)]=2[9+2√5-4]=2[5-2√5]= 10-4√5。 For reference only.