Even though there are a lot of lines and "rectangles" in this building, the equal sign stands out because it only has two lines that are the same size. Whereas the other lines are in clusters of 5 or more. The black and white helps draw the eye into the negative white space. it also helps that the equal sign is not smack in the center, but to the right. The eye naturally starts at the upper left corner and immediately travels to the equal sign.