s = sqrt { [ 9 + 1 + 1 + 9 ] / 3 } = sqrt (20 / 3) = sqrt ( 6.67 ) = 2.58

how they get the 2.58

By using a calculator that is the only way people take square roots. Before calculators, there was a way to figure it out by hand, but it was long, tedious and confusing.