\(7x^2-35x+42=7x^2-14x-21x+42\)
\(=7x\left(x-2\right)-21\left(x-2\right)=\left(7x-21\right)\left(x-2\right)\)
đa thức có nghiệm <=> \(\left(7x-21\right)\left(x-2\right)=0\Leftrightarrow\int^{7x-21=0\Rightarrow x=3}_{x-2=0\Rightarrow x=2}\)
Vậy..........................