This is simply what I wrote for my assignment, sorry I am a bit late.
<em>It was confusing when she took out the negative and postive pairs, but then I soon knew that the negative tiles are like subtracting positive tiles. Then once she got the total I was amazed with how easy it was to find out the value of x.</em>
let's say the Eastbound train is going at a rate of "r" mph, since we know the Westbound train is going faster 12 miles more than that, then the Westbound train must be going "r+12".
we also know that after 4 hours, both of them were 760 miles apart, so