Arlen,
Try it with numbers first keeping track of what you do.
Suppose the length is 12 inches, that is L = 12 inches.
The width is 8 less than the length so the width is 12  8 = 4 inches.
The area of the triangle is length times width which is 12 × 4 = 48 square inches.
Now again but without the specific value for L.
Suppose the length L inches.
The width is 8 less than the length so the width is L  8 inches.
The area of the triangle is length times width which is L × (L  8) square inches.
I hope this helps,
Penny
