Perimeter is length + length + width + width, or 2l + 2w. 84 = 2l + 2w. The length = 2w + 6.
Substitute the length value into the first equation. 84 = 2(2w + 6) + 2w; distribute
84 = 4w + 12 + 2w; combine like terms
84 = 6w + 12; subtract 12 from both sides
72 = 6w; divide both sides by 6
12 = w so the width is 12
If the length is 6 meters longer than twice the width and twice the width is 24, 24 + 6 = 30 as the length
You can check it by adding. 12 + 12 + 30 + 30 = 84 so it's right!