it's x=1
the equation for the vertical line passing through (1,0)
if you need demo you calculate the mean for every pair of corners of those triangle
ex: for the upper corner you have coordinates (-1,3) on the left one and (3,3) for the right one
the mean of those points gives you (1,3)
for the lower corner you have left (-2,-1) and right (4,-1) that gives you the mean (1,-1)
if you already know that this is a symetry situation you only need 2 points to define the symetry axis wich is obviously a line (you only need 2 points to define a line)
and this line is [y = anything, x=1]