17) The wording is not the best, but they want you to do 1/2 + 1/4, which is 3/4.
18) a). vertixes are the pointy bits, the ones at the angles of the faces, and edges connect vertices (count them: there are 8 vertices and 12 edges). Imagine going from one edge to the one farthest from it. If you have playdoh or some breadto cut, try it. You'll see that the section you'll obtain is a rectangle, if what you cut looks like the figure.