The area of a triangle is equal to the base times the height divided by two. In this case, the base is 6 (or x, it doesn't really matter) and the height is x (or 6, it doesn't matter). That means that the area of this triangle is equal to 6 * x divided by 2, which is also equal to 18. we can say that 6x/2 = 18, meaning that 6x = 36, so x = 6.
