Explanation:
the inside expression of an absolute value expression can be positive or negative, but the result is only the positive one.
therefore, for our example here the negative case would be
2.5x - 6.8 = -12.9
which gives us
2.5x = -6.1
x = -6.1/2.5 = -2.44
and the positive case would be
2.5x - 6.8 = 12.9
and that gives us
2.5x = 19.7
x = 19.7/2.5 = 7.88