\[\left( (a - b) - c \right)^{2} =\]
\[= (a - b)^{2} - 2c(a - b) + c^{2} =\]
\[= a^{2} - 2ab + b^{2} - 2ac + 2bc + c^{2} =\]
\[= a^{2} + b^{2} + c^{2} - 2ab - 2ac + 2bc.\ \]