In order to calculate the coordinates of the points after a translation of 3 units right and 1 unit down, let's add 3 units to the x-coordinate and decrease 1 unit from the y-coordinate of all points.
So we have:
Then, to calculate the coordinates after a reflection across the y-axis, let's change the signal of the x-coordinate of all points.
So we have:
Drawing the resulting triangle, we have: