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