We are given:
Area = 21 inches²
Length = 6 inches
Width = x inches
We can use this formula to solve for the width:
Area = Length × Width
21 inches² = 6 inches · x inches
21 inches² = 6x inches²
21 = 6x
Now, divide both sides by 6.
21/6 = 6x/6
21/6 = x

The width is
3.5 inches.