Thanks for your comment, it has been helpful.
But I would like to know why did they use (2x/m) as its height? And I would like to break that question into two
1) having ax on the small triangle's base implies that there's a relationship with the base and it's height. What is the relationship? Isn't it two separate events?
2) Similar question to "m" how does the "m" in the height have any relation with the "m" that's being used for amount of crops that can grow in a sqft.
Because if the height was replaced with a "y", the whole formula wouldn't exist eventho it may or may not equals to 2b/m.