[tex]i = \sqrt{-1} [/tex]
This is the fundamental property of i that you'll need to keep in mind when solving any problem with imaginary numbers (5i, 2i, 18i, etc.) or complex numbers (3+2i, 8-5i, etc). Squaring both sides of this definition, we also get that
[tex]i^2=-1[/tex]
Which we can use to solve this problem. We have:
[tex](-2i)(5i)(-1)\\
=(-10i^2)(-1)\\
=(-10)(-1)(-1)\\
=-10[/tex]
