Get values in complex numbers problem

Question

I have a problem, and I have advanced but I do not know how to get to the solution, which is $-2$

It says:

If $3i + 5i^2 = a + bi$, so $a + b$ = ?

I need the value of $a + b$ given the data

I got here:

Image of my resolution

In case you can not see it, my development was in:

$16 = (a + b)(a - b)$

Answer 1

Remember that $i = \sqrt{-1}$. Thus $i^2 = -1$. That means:

$$3i + 5i^2 = 3i + 5(-1) = 3i - 5 = -5 + 3i$$

So $a = -5$ and $b = 3$, and $-5 + 3 = -2$.