Now the width is w.
It's twice as long as wide, so now the length is 2w.
If the length is increased by 4 cm, the length will be 2w + 4.
The width is decreased by 3 cm, so the width will be w - 3.
The are of the new rectangle is 100 cm^2.
area = length * width
area = (2w + 4)(w - 3)
The area of the new rectangle is 100, so we get
(2w + 4)((w - 3) = 100
2w^2 - 6w + 4w - 12 = 100
2w^2 - 2w - 112 = 0
w^2 - w - 56 = 0
(w - 8)(w + 7) = 0
w - 8 = 0 or w + 7 = 0
w = 8 or w = -7
A width cannot be negative, so discard w = -7.
w = 8
The width is 8 cm.
The length is twice the width, so the length is 16 cm.