Firstly, we are given the points.

The points are taken from a function f.
Let's assume the function is;

The coordinates of these three points are taken through horizontal stretch by a factor of 3.
For an horizontal stretch of a factor of 3, the function becomes;

So, to attain the same value of y, x values need to be trippled.
That means all x coordinates would be multiplied by 3.
So, we then have the coordinates;

Then it was followed by a reflection across the x-axis.
For reflection across the x-axis, the y coordinates would be multiplied by -1.

The coordinates will then become;

So, the coordinates of these three points after a horizontal stretch by a factor of 3, followed by a reflection across the x-axis is;