What is the matrix for a reflection across the xxx-axis?
\\begin{bmatrix} 1 & 0 \\\\ 0 & -1 \\end{bmatrix}
\\begin{bmatrix} -1 & 0 \\\\ 0 & 1 \\end{bmatrix}
\\begin{bmatrix} 0 & 1 \\\\ 1 & 0 \\end{bmatrix}
\\begin{bmatrix} 1 & 0 \\\\ 0 & 1 \\end{bmatrix}