Gigabyte89 said:

How can you get -3, but you just left the second number as 15?

I got -3 x 15 = -45 also. The reason you leave the second number as 15 is because if a 2's compliment number begins with 1, you know it is negative. Otherwise if it begins with a zero it's just treated as a normal positive binary number.

The first 2's compliment number is 11111101 --> we know it's negative because it begins with 1.

To turn it into its positive equivalent, we first turn it into 1's compliment by reducing it by 1, making it 11111100.

Then the 1s and 0s are reversed to turn the number from a 1's compliment to its positive equivalent.

--> ie. 11111100 is changed to

00000011

= 3, but remembering that the number is actually negative, it's -3.

so the result is -(00000011) x 00001111

= -3 x 15

= -45 in base 10.