The linear transformation is given as $\displaystyle T \begin{bmatrix}x \\ y \end{bmatrix} = \begin{bmatrix}\ x+y \\ x-y\end{bmatrix}$ and I have to show the effect on bothe the unit square as well as a triangle with vertices (-1,0); (0,1); (1,0).

As far as it's effect on the unit circle would it simply a reflection around the x-axis (to account for the -y in x-y) as well as a shift up by y in the x direction? ( I know that doesn't quite make sense..)

For the triangle I'm even more confused, would it still be a refection over the x-axis, but would it also lead to some sort of horizontal shear?