Answer
$20$
Work Step by Step
Step 1: $4\textbf{i}$ is basically $4\textbf{i}+0\textbf{j}$ which means it can be written as $\langle 4,0 \rangle$. On the other hand, $5\textbf{i}-9\textbf{j}$ can be written as $\langle 5,-9 \rangle$.
Step 2: $\langle 4,0 \rangle \cdot \langle 5,-9 \rangle$
Step 3: $4(5)+0(-9)$
Step 4: $20+0$
Step 5: $20$
