This is LaTeX code:
[latex]
$s_b (x, y) = (-x, y) \\ s_a (x, y) = (x, -y) \\ s_C (x, y) = (-x, -y) \\ (s_a \circ s_b)(x, y) = s_a(s_b(x, y)) = s_a(-x, y) = (-x, -y) = s_C (x, y)$
[/latex]