Question

How do you simplify (-4i)(7i)?

Answer 1

$\bf (-4i)(7i)\implies -28i^2\implies \stackrel{\textit{recall that }i^2 = -1}{-28(-1)}\implies 28$

Answer 2

For this case we must simplify the following expression:

$(-4i) (7i)$

We have to by definition that:

$- * + = -\\- * - = +\\i ^ 2 = -1$

So:

$-4i * 7i = -28i ^ 2 = -28 (-1) = 28$

Finally we have that the expression is equivalent to:

$(-4i) (7i) = 28$

Answer:

$(-4i) (7i) = 28$