To translate a triangle is to move every point of it along a certain vector.
So in your case, they want you to translate it by -2 units in the x axis, and 4 units in the y axis.
This means you have to move each point 2 units to the left, and 4 units up.
So A' is (3+(-2), 4+4) which is (1, 8)
repeat for the others and you will get
B' is (2, 6)
C' is (-1, 6)
Hope this helps