From the picture, DE is congruent with HG, that is,
DE = HG
Substituting with data,
5x - 1 = 4x
4x is adding on the right, then it will subtract on the left
5x - 1 - 4x = 0
1 is subtracting on the left, then it will add on the right
5x - 4x = 0 + 1
x = 1
Therefore, the length of DE is:
DE = 5x - 1
DE = 5(1) - 1
DE = 5 - 1
DE = 4