The product on multiplying - 4bc with 2a is - 8abc. That is, a term with minus sign multiplied with a term having a positive term gives a product which has a minus sign.
On the whole we have to remember these rules while multiplying quantities, which are the same as above, but repeated here due to its importance,
| + |
x |
+ |
= |
+ |
| - |
x |
- |
= |
+ |
| - |
x |
+ |
= |
- |
| + |
x |
- |
= |
- |
(Note: The multiplication of binomials is shown)