AB + BC = AC because B is a point on the line AC, so AB and BC are line segments on the line AC. The point B creates two line segments out of line AC.
2x + 6x + 8 = 32 because the two line segments are 2x and 6x + 8, adding them together will give you 32, the length of the entire line AC.
8x + 8 = 32. This is just adding together the x's into a single object.
8x = 24 is subtracting 8 from both sides to get rid of the 8 on the side with x, to start isolating the x.
x = 3 because you divide 24 by 8 to completely isolate x on one side of the equation.