a, \(3\left(x-1\right)+5\left(x-1\right)=8\left(x-1\right)\)
b, \(x^2-2xy+y^2-25=\left(x-y\right)^2-25=\left(x-y-5\right)\left(x-y+5\right)\)
c, \(x^2+2xy+y^2-16a^2=\left(x+y\right)^2-\left(4a\right)^2=\left(x+y-4a\right)\left(x+y+4a\right)\)
d, \(x^2-5x-6=x^2-6x+x-6=x\left(x-6\right)+\left(x-6\right)=\left(x+1\right)\left(x-6\right)\)