ANSWER :
A 180 degrees rotation about the origin.
EXPLANATION.
Let us name the vertices of the triangle as follows.

If we reflect the triangle across the y-axis, then we have to negate the x-coordinates
to get,

If we now reflect triangle ∆A'B'C' in the x-axis, we will have to negate the y-values to obtain,

The single transformation that will map ∆ABC to ∆A''B''C'' is

This is a rotation of 180 degrees about the origin.