Additional to expressions with \(i\) in the formula appears \(i^2\).
We can easily replace this \(i^2\).
By definition \(i\) is \(i^2 = -1\).
So we replace \(i^2\) with the number \(-1\) and continue to calculate with the result from above as usual.
The result of the calculation is \(5 - 5i\).
This article describes the multiplication of complex numbers in normal form.
Easier to calculate is the Multiplication of complex numbers in polar form.