To simplify the expression \( -2i(3i + 4i) \), we first combine the terms inside the parentheses:
\[ 3i + 4i = 7i \]
Now, substitute this back into the expression:
\[ -2i(7i) \]
Next, we multiply:
\[ -2i \cdot 7i = -14i^2 \]
Since \( i^2 = -1 \), we substitute \( -1 \) for \( i^2 \):
\[ -14i^2 = -14(-1) = 14 \]
Thus, the expression simplifies to:
\[ 14 \]
So the equivalent expression is \( \boxed{14} \).