Help! This one step for solving standard deviation I don't understand...

When I got it wrong it tells me what to do but I don't get it? (2nd image)

Help This one step for solving standard deviation I dont understand When I got it wrong it tells me what to do but I dont get it 2nd image class=
Help This one step for solving standard deviation I dont understand When I got it wrong it tells me what to do but I dont get it 2nd image class=