Equilateral triangle means that all the sides equal the same.
So
6x = 4x+8 = 3x+12
now we choose two sides and solve for x
6x = 3x + 12
subtract 3x from both sides
3x = 12
divide both sides by 3
x = 4
Now we can substitute it in to check
6x = 4x+8 = 3x+12
6(4) = 4(4) + 8 = 3(4) + 12
24 = 16 + 8 = 12 + 12
24 = 24 = 24
Hope this helps :)