[tex]\begin{gathered} \text{Given} \\ (3i)(9i) \end{gathered}[/tex][tex]\begin{gathered} \text{Multiply the coefficients first, and next is the imaginary number} \\ (3i)(9i)=3\cdot9\cdot i\cdot i \\ =27i^2 \end{gathered}[/tex][tex]\begin{gathered} \text{recall that }i^2=-1 \\ \text{Substitute it to the product from before and we get} \\ =27i^2 \\ =27(-1) \\ =-27 \end{gathered}[/tex][tex]\begin{gathered} \text{Therefore,} \\ (3i)(9i)=-27 \end{gathered}[/tex]
