# How do you multiply  (2-i)(1+7i)  in trigonometric form?

simplify your equation and then use the following formulae: z=a+bi=|z|(cosθ+isinθ) and $| z | = \sqrt{{a}^{2} + {b}^{2}}$
After factoring out your function $\left(2 - i\right) \left(1 + 7 i\right)$ you will get $2 + 13 i - 7 {i}^{2}$. Since ${i}^{2} = - 1$ we get $2 + 13 i + 7$. Then we get $9 + 13 i = z$. $a = 9$ and $b = 13$. You should be able to do the calculations from there. Good luck.