# How do you multiply  (-10+7i)(-9+2i)  in trigonometric form?

$90 - 20 i - 63 i + 14 {i}^{2} \to 90 - 83 i - 14 = 76 - 83 i$
Foil it out and the collect like terms. Note that ${i}^{2} = - 1$ so I substitute it in and simplify.