Question 843712
Use the FOIL Method,
{{{ (3a+7)(3a-7)=3a(3a)-7(3a)+7(3a)-7(7)}}}
{{{(3a+7)(3a-7)=9a^2-21a+21a-49}}}
{{{(3a+7)(3a-7)=9a^2-49}}}