a: \(\left(x+y+z\right)^2-\left(y+z\right)^2\)
\(=\left(x+y+z-y-z\right)\left(x+y+z+y+z\right)\)
\(=x\left(x+2y+2z\right)\)
b: \(\left(x-3\right)^2-2\left(x^2-9\right)+\left(x+3\right)^2\)
\(=\left(x-3-x-3\right)^2\)
=36
c: \(\left(a^2-b^2\right)^2-\left(a+b^2\right)^2\)
\(=\left(a^2-b^2-a-b^2\right)\left(a^2-b^2+a+b^2\right)\)
\(=\left(a^2-a-2b^2\right)\left(a^2+a\right)\)
\(=a\cdot\left(a+1\right)\left(a^2-a-2b^2\right)\)