On pages 128,129 in the Principal Curvature section of his text Kreyszig states the series of mathematical steps copied in the graphic attached. Beginning with and beyond the differentiation step I cannot follow his mathematics. For example, where does the minus sign come from between the parentheses? Each step condenses many intermediate steps, particularly because of the indices. His expression incorporating the kronecker delta is particularly puzzling. Anyone who can expand -or deconstruct one might say- the mathematical steps would be greatly appreciated.