The answer to this question is the Distributive Property. Remember that "distributing" means to share out. In this case, we are distributing -4 into each term within the parenthesis. Let's further evaluate on that; -4(x+ 3); -4(x) + -4(3); -4x - 12.
As we can see, we multiplied -4 by each term within the parenthesis (as we did not have any sign between our number and the parenthesis, which concludes we need to multiply that term).