Because is a binomial * binomial the term in the first bracket have to be multiplied by the term in the second bracket. And this term it gives 2*2=4 terms (3+7v^2)*(3-7v^2)= 9-21v^2+21v^2-49v^4 Because the middle terms(21v^2+21v^2) are additive inverses we delete them and we remain with 9-49v^4. I hope I helped u:)
