Imagine a long timeline with 0 in the middle, on the left we have -356.1 and on the right we have 356.1. Adding numbers makes the timeline move right.
Starting from the left, add (+) 258.6 to the number (moving right along the timeline). As 356 is larger than 258, the scale does not reach 0 yet and therefore the number must be negative.
To find the total area of the octagon you can find the area of the rectangle that is created by the entire figure and subtract the areas of the four congruent right triangles.
(10 cm x 6 cm) - [4(1/2 x 2 x 2)] 60 cm² -8 cm² = 52 cm²
