Simplify the expression. Please show your work (3i)(7i)

To simplify the expression (3i)(7i), we can use the property of multiplying imaginary numbers:

(i)^2 = -1

By substituting this value into our expression, we have:
(3i)(7i) = 3 * 7 * (i)^2

Now we can substitute (i)^2 with -1:
(3i)(7i) = 3 * 7 * (-1)

Multiplying the numbers, we get:
(3i)(7i) = -21