Answer
See answer below
Work Step by Step
Let:
$A_1=\begin{bmatrix}
1 & 1\\
0 & 1
\end{bmatrix}$
$A_2=\begin{bmatrix}
2 & -1\\
0 & 1
\end{bmatrix}$
$A_3=\begin{bmatrix}
3 & 6\\
0 & 4
\end{bmatrix}$
We can see that $5A_1+(-1)A_2=5\begin{bmatrix}
1 & 1\\
0 & 1
\end{bmatrix}+(-1)\begin{bmatrix}
2 & -1\\
0 & 1
\end{bmatrix}=\begin{bmatrix}
3 & 6\\
0 & 4
\end{bmatrix}=A_3$
Hence $\{ A_1,A_2,A_3 \}$ is a linearly dependent set in $M_2(R)$
