Expression: $\left( -3i \right) \times \left( 6i \right)$
Multiplying a negative and a positive equals a negative: $\left( - \right) \times \left( + \right)=\left( - \right)$ $-3i \times 6i$
Calculate the product $-18{i}^{2}$
By definition ${i}^{2}=-1$ $-18 \times \left( -1 \right)$
Any expression multiplied by $-1$ equals its opposite $18$