Not sure need help with Subtracting fractions.

Hi I'm Domino and I'm not sure if this is where my question goes. I have started college and i'm in an algebra 03 class. Hope you can help. This is as far as I've gotten.
3 1/8-1 5/12=?
25/8-17/12=?

Awesome!! Now find a common denominator (the LCD is preferred), and add away.

Okay I'm kinda brain dead right now. How do I find the LCD? what I did was factor them and it didn't seem to help. Here are my factors for both the 8 and the 12
8
/\
2 4
/\
2 2

12
/\
2 6
/\
2 3

Now i'm stuck or have I done that complete wrong?

25/8-17/12 = ??

Find a number (any number but preferably the lcd) that both 8 and 12 will "go into". Suppose you choose 120 (there are better ones). Then build equivalent fractions having the same denominator.
\(\displaystyle \frac{25}{8}=\frac{25}{8}\cdot \frac{15}{15}= \frac{375}{120}\)

\(\displaystyle \frac{17}{12}=\frac{17}{12}\cdot \frac{10}{10}=\frac{170}{120}\)

Now, subtract and reduce the result to lowest terms.

In the mean time, to make your work easier, learn how to find the least common denominator.