Multiplying two imaginary numbers then simplifying the expression to a real number

The video tutorial explains how to multiply two imaginary numbers by treating them as monomials. It covers the multiplication of coefficients and the concept of I squared, which equals -1. The final result is obtained by multiplying the product of the coefficients with -1, resulting in a positive number.

How do you simplify the expression I squared when dealing with imaginary numbers?

What is the final result when multiplying -21 and -1?