It often works well to separate the constant term from the others, then complete the square by adding the square of half the linear term coefficient.
w² + 6w = 11
w² + 6w + 3² = 11 + 3² . . . . add the square of half of 6
(w +3)² = 20 . . . . . . . . . . . . simplify
w + 3 = ±√20 . . . . . . . . . . . take the square root
w = -3 ± 2√5 . . . . . . . . . . . .subtract 3, simplify