I came across a math related problem in my science text book I can't figure out on my own.

The first part of the problem has led me to this equation:

I am 100% positive that the first part is right but the second part of the problem requires the equation to be in terms of Y.

The textbook has an answer in the back of the book, but so far I haven't been able to figure it out. There are a couple of problems like this so if someone could help me figure out this problem it would be a huge help.

The details: the answer in the back of the book reads