Consider Ashley's pennant to have the following shape
In our case, b=8 inches. The area of a triangle of base b and height h is given by the expression

Since b=8, we get

Now, this expression represents the area of the triangle. We are told that this area should be at least 28 squared inches, then we have the inequality

If we divide by 4 on both sides, we get

So the height of the pennant should be at least 7 inches