You have to use the Pithagorean Theorem because the ladder, the barn and the earth forms triangle. Than the wall of the barn and the earth makes a 90° angle. So the ladder is the hypotenuses and the earth and the barn wall are the catetes so you only have to do 12^2= 3^2 + x^2. x is how high on the barn the ladder reach. That's all. I hope I helped.