The property used in the equation \(-3(x+5) = -3x - 15\) is the Distributive Property.
The Distributive Property states that for any numbers \(a\), \(b\), and \(c\):
\[ a(b + c) = ab + ac \]
In this case, we can apply it as follows:
\[ -3(x + 5) = -3 \cdot x + (-3) \cdot 5 = -3x - 15 \]
Thus, the equation is correctly applying the Distributive Property to expand the left side.