Folding only adds numbers together, so the total never changes. Both strips add up to 30. Your largest target number, 21, must be a stack of several cells added together.
Hint 2: the length budget
The strip has 8 cells and the target has 3, so the folds together must remove 5 cells. A fold removes as many cells as its shorter side has.
Hint 3: the first fold
The first fold goes between cell 6 and cell 7, counting cells from the left.
Full solution
Show every fold
Fold 1 of 3
71118282→71111010
Fold between cell 6 and cell 7. The right side flips over the left side.
Fold 2 of 3
71111010→711120
Fold between cell 5 and cell 6. The right side flips over the left side.
Fold 3 of 3
711120→7212
Fold between cell 3 and cell 4. The right side flips over the left side.