@HotStuff68 If I understand your question correctly, you are asking about map f(x,y)=(x,-y).

I.e. point with old coordinates (x,y) has new coordinates (x,-y).

Matrix of such map would be [[1,0],[0,-1]].

$$\begin{pmatrix}1&0\\0&-1\end{pmatrix}$$

In any case, this would be more suitable for math.SE, which is for lover level than MathOverflow. There even is a linear algebra chatroom on teat site. (Although it is not very active.)