\(\left(2x^2+6x+1\right)^2-26x^2\left(2x^2+6x+1\right)+26x^2\left(2x^2+6x+1\right)-26x^2\)
\(-26x^2\left\{\left(2x^2+6x+1\right)^2-\left(2x^2+6x+1\right)+\left(2x^2+6x+1\right)\right\}\)
\(-26x^2\left\{\left(2x^2+6x+1\right)\left(2x^2+6x+1\right)-1+1\right\}\)
\(-26x^2\left(2x^2+6x+1\right)\left(2x^2+6x+1\right)\)
\(-26x^2\left(2x^2+6x+1\right)^2\)