Answer
$-4$
Work Step by Step
Step 1: $2\textbf{i}+4\textbf{j}$ can be written as $\langle 2,4 \rangle$. On the other hand, $-\textbf{j}$ is basically $0\textbf{i}-1\textbf{j}$ which means it can be written as $\langle 0,-1 \rangle$.
Step 2: $\langle 2,4 \rangle \cdot \langle 0,-1 \rangle$
Step 3: $2(0)+4(-1)$
Step 4: $0-4$
Step 5: $-4$