Answer:
Equation:

Width = 4 feet
Length = 5 feet
Explanation:
Let x feet be the width of the rectangle.
If the length of the sandbox is one foot longer than the width, then the length is x + 1 feet.
Use formula

The perimeter of a childs rectangular sandbox is 18 feet, thus
![18=2(x+(x+1))\\ \\18=2(x+x+1)\\ \\18=2(2x+1)\\ \\9=2x+1\ [\text{Divided by 2}]\\ \\2x+1-1=9-1\\ \\2x=8\\ \\x=4\ feet \\ \\x+1=5 \ feet](https://img.qammunity.org/2020/formulas/mathematics/middle-school/41zsq9o028fwtfc6sandljti81fgwp79od.png)