In order to calculate this product, we need to use the distributive property of multiplication:
[tex](a+b)(c+d)=a\cdot c+a\cdot d+b\cdot c+b\cdot d[/tex]So we have:
[tex]\begin{gathered} (3rs-7)(3rs-4) \\ =3rs\cdot3rs+3rs\cdot(-4)+(-7)\cdot3rs+(-7)\cdot(-4) \\ =9r^2s^2-12rs-21rs+28 \\ =9r^2s^2-33rs+28 \end{gathered}[/tex]