Let's remember what multiplication means:

Remember that

3 x 4 means three 4's added together

3 x 4 = 4 + 4 + 4 = 12

So, what if we stick a negative number in there?

3 x -4 =

We can do the same thing:

3 x -4 means three -4's added together.

3 x -4 = -4 + -4 + -4 = -12

3 x -4 = -12