I realise this is quite late but in case you still wanted the answer, the width is 3m.
If the length is 3m longer than the width, you can write the width as x and the length as x + 3. The perimeter would be both lengths and both widths added together, so you would just write it as:
x + x + x + 3 + x + 3 = 18
4x + 6 = 18
- 6
4x = 12
÷ 4
x = 3
I hope this helps!
