Answer
$$
\begin{bmatrix}
7\\
8
\end{bmatrix}
$$
Work Step by Step
To compute this product
$$
\begin{bmatrix}
8 & 3 & -4 \\
5 & 1 & 2
\end{bmatrix}
\begin{bmatrix}
1\\1\\1
\end{bmatrix}
$$
We can use the definition of matrix-vector multiplication. Since all the entries of the vector are $1$, we get the following product:
$$
\begin{bmatrix}
8+3-4\\
5+1+2
\end{bmatrix}
$$
Which simplifies to
$$
\begin{bmatrix}
7\\
8
\end{bmatrix}
$$
