Hey, so basically this just dealing with similar triangles and using proportions.
You know that the server is 8 feet high, and the net is 3 feet high. Assuming the server serves the ball linearly, it clears just above the net. You know that the distance the server is from the net is 40 feet. So if you draw your triangle:
You're trying to find the overall distance, which I've labelled as L. So then the distance from where the ball lands and the net is considered L-40. The property of similar triangles is basically using ratios, where the smaller triangle is proportional to the larger triangle (or vice versa). So if you wrote it out, you would have:
3/(L-40) = 8/L
The ratios can be switched around, but have to "match" as they are proportional (ie. if you write length/height for one ratio, do so for the other ratio).
Through cross-multiplication, you can now solve for L (I'm assuming you can do algebra here) to get L = 64 feet. (Edit: Whoops, it wants how far the ball is from the net, so just take L-40 = 24 feet.)
Hope that helps, I was bored lol. IMO the most important part is knowing how to draw the triangle. The rest is a piece of cake.
If my post wasn't too clear, you can also have a look-see here: http://www.math.com/...s/S1U2L4DP.html (I just googled it, but it should help if you don't understand)
Yes, it proved helpful. Thank you