The rule for translating a point across the x-axis is:
(x,y) ---> (x, -y)
For instance, the point A on your triangle would translate as follows:
A ---> a
(1,4) ---> (1,-4)
Can you do points B and C? For more information on geometric transformations, check out the following Wyzant file:
https://www.wyzant.com/resources/files/629425/geometric_transformations_revised_11_2017