The perimeter is defined as the sum of all the side lengths, and a rectangle has two (possibly different) side lengths - let's call them length and width. There are two lengths, each one is 8.3, so the lengths make 16.6 cm.
We need the two widths, and we have perimeter = 2 lengths + 2 widths. Or 22.4 = 16.6 + 2 widths.
2 widths = 22.4 - 16.6 = 5.8.
1 width is half of 5.8, or 2.9 cm.
Quick check: 22.4 = 8.3 + 8.3 + 2.9 + 2.9