[tex]\bf 3\sqrt{7}-\sqrt{7}=3\implies 3\sqrt{7}-1\sqrt{7}=3\implies \stackrel{\textit{adding like-terms}}{2\sqrt{7}\ne 3}[/tex]
bear in mind that 3√7, is really 1√7 + 1√7 + 1√7, if we subtract 1√7 from it, we're left with 2√7.
