You can use the cosine rule for this - you might have to change some of the letters around for it to make sense though. Hope this helps!
The answer is c because you plug in the values given. 4 times .75 is three. Three plus three is 6. For the other side you plug in .50. 6 times .50 is 3. 3-2 is 1. Then you are asked to find the perimeter, a rectangle has 4 sides so you do 6 on the top and bottom and 1 on each of the sides. When you add those all together you get 14 which is c.
The surface area of a cube is the combined area of all of its faces. The faces of a cube are squares, so to find the area of one face, we simply need to multiply the side length, which is 20, by itself.
20 * 20 = 400
So one face has an area of 400 units squared. To find the surface area, we just need to add all of the individual areas together. Cubes have 6 faces, so we can just multiply 400 by the amount of faces there are.
400 * 6 = 2400
The net, when folded into a cube, has a surface area of 2,400 units squared.
Hope that helped! =)
