If the perimeter is 28cm then the length and width add up to half this, which is 14cm. Since the length is 10cm more than the width, we subtract 10cm from 14cm to give us 4cm, which is the equivalent of two widths, so the width is 2cm and the length is 10cm more, making the length 12cm.
If we let w=width then length=w+10.
The perimeter is 2(length+width)=28, so 2(w+10+w)=28.
The equation is 2(2w+10)=28.
To solve it, we halve both sides:
2w+10=14, 2w=14-10=4 (subtract 10 from each side), so w=4/2=2cm (halve both sides). And length=w+10=2+10=12cm.