The perimeter of a rectangle is 42 inches. If the width of the rectangle is 6 inches, what is the length?

Let L be the length of the rectangle.

The perimeter of the rectangle is 2L + 2W = 42.
Substituting the given width, we get 2L + 2*6 = 42
Combining like terms, we get 2L + 12 = 42
Subtracting 12 from both sides, we get 2L = 30
Dividing both sides by 2, we get L = <<15=15>>15 inches. Answer: \boxed{15}.