The width of a rectangle is the length minus 3 units. The area of the rectangle is 18 square units. What is the width, in units, of the rectangle?

Let the length of the rectangle be L units.

The width of the rectangle is L - 3 units.

The area of the rectangle is given by:
Area = length x width
18 = L(L - 3)

Expanding the equation and setting it equal to zero:
18 = L^2 - 3L
0 = L^2 - 3L - 18

Now we need to solve this quadratic equation to find the length of the rectangle:

(L - 6)(L + 3) = 0
L = 6 or L = -3

Since the length of a rectangle cannot be negative, the length of the rectangle is 6 units.

Therefore, the width of the rectangle is:
Width = Length - 3
Width = 6 - 3
Width = 3 units

So, the width of the rectangle is 3 units.