Amanda leaned a 15-foot ladder against the top of her house so she could get on the roof, which is 12 feet from the ground.

If you're trying to find the missing side of the "triangle" (the distance from the house to where the ladder touches the ground), you would use the pythagorean theorem, a²+b²=c². (c is always hypotenuse)

a²+b²=c²

12²+b²=15²

144+b²=225

b²=81

√b²=√81

b=9