Don't look at this comment.. I messed up the first time so, yeah.
The cheap answer is, you simply "grab the denominator of one and multiply it times the other's top and bottom", so let's do so,
Here are some very good video that explain how. very short and easy too. http://virtualnerd.com/pre-algebra/linear-functions-graphing/rate-of-change-slope/finding-slope/slop... math antics on youtube is great for help too