a) This I can do:

b) My working is as follows:

When

Now how do I get rid of the mod signs to get their form? I did the following:

But then how to get rid of the mods in the denominator? Since it still could be negative and you need the mods to ensure it's positive?

Next part says:

When

Solving these on the TI-89 gives:

Now since the question asks for the speed we require

However which one do you choose?

Now if you solve their equation which is

you get only and not

So which formula do you use and most importantly when you use my formula how do you dismiss as a solution?