To add the monomials \( 15AB^3 \) and \( -(-15AB^3) \), we can first simplify the expression:
\[ -(-15AB^3) = 15AB^3 \]
Now, we can add the two terms together:
\[ 15AB^3 + 15AB^3 = 30AB^3 \]
So, the result of adding \( 15AB^3 \) and \(-(-15AB^3)\) is:
\[ 30AB^3 \]