I'm pretty sure when they sought out ways to derive a quadratic equation to help them reason triangular regions they had to think frontwards and backwards. First, I believe you need to understand how the height 2x/m came into play (why it was used). First, keep in mind that "m" represents a basic unit of 1. That would mean that 2x/m (the height for BOTH triangles would appear to be 2x. But are their heights 2x? Let's think about it, when finding the area of a right triangle we eventually divide the area by 2 after multiplying the b x the h. Knowing this, it is mathematically reasonable why the coefficient of "2" was put in front of x-it would get divided back out and preserve what they really wanted for the height of the triangle/ length of one side of the land "x". This mean that the small triangular area would be ax times x or (ax^2). The larger triangular area would be b times x or bx for its area. You asked though "what is "a" and "b"? look at length of base "a", compared to the triangles height that we previously deduced to really be "x". "a" represents a coefficient thats taking a fraction of base length "x" for the small base is being represented in terms of the height of the triangle or length of the land. Base b ls obviously the second width of the scalene triangle or width of land that IF represented with bx instead of b (like it is) would have created a bx^2 term instead of the bx we need to figure out the area the land in addition to other things. This is my perception after being confused there for a minute too. I hope this helped you or someone just a little although it's years later- just discovered this awesome forum:).