Answer
Valid by universal modus tollens.
Work Step by Step
Universal modus tollens: $\forall x, $ if $P(x)$ then $Q(x)$.
~$Q(a)$ for a particular $a$.
~$\therefore P(a)$.
In this case P(x) is: the product of two numbers is 0.
Q(x) is: at least one of the numbers is 0.
a are the particular two numbers (2x+1) and (x-7) for some number x.
