\(\left(a+b-c\right)^2+\left(a-b+c\right)^2-2\left(b-c\right)^2\)
\(=\left[\left(a+b\right)-c\right]^2+\left[\left(a-b\right)+c\right]^2-2\left(b-c\right)^2\)
\(=\left(a+b\right)^2-2\left(a+b\right)c+c^2+\left(a-b\right)^2+2\left(a-b\right)c+c^2-2\left(b^2-2bc+c^2\right)\)
\(=a^2+2ab+b^2-2ac-2bc+c^2+a^2-2ab+b^2+2ac-2bc+c^2-2b^2+4bc-2c^2\)
\(=a^2+2ab+b^2-2ac-2bc+c^2+a^2-2ab+b^2+2ac-2bc+c^2-2b^2+4bc-2c^2\)
\(=2a^2\)