We know that these products are
[tex]\begin{gathered} (x-3)^2=x^2-6x+9 \\ (x+3)(x-3)=x^2-9 \end{gathered}[/tex]The first product has three terms because the square power of a binomial will always have three terms given that it follows the following rule
[tex](a-b)^2=a^2-2ab+b^2[/tex]The second product has just two terms because it's not the binomial to the square but it's a perfect square difference that follows the following rule
[tex](a+b)(a-b)=a^2-b^2[/tex]