OK - I'm bored tonight.
It's a linear relationship...
1. draw the x axis going from 0 to 200000 lbs, marked every 5000 lbs
2. draw the y axis going from 0 to, say, 20000 ft in 1000 ft increments.
3. the result is a graph w/ 40 units in the x axis, and 20 units in the y axis.
4. plot (for the jt8d-7) the points (110000, 12000) and (145000, 2000).
5. we're talking about a linear relationship, so determine the slope (delta y/delta x, which = 10000/-35000)
6. so the slope = -.2857. (common sense check, negative slope means the alt capability decreases as the weight increases, so that's good)
7. use the point slope formula. (y-y1)=m (x-x1)
8. (y-12000) = -.2857(x-110000)
9. solve for y
10. y= -.2857x + 43427
11. plug in x=145000 to check the math, should be 2000.
12. 2000.5. close enough (rounding errors)
13. So for any weight b/t 110,000 lbs (about as light as a functional 727-200adv can be) and 200,000 lbs, plug it in for x, and the equation in item 10 above spits out the altitude a -7 powered a/c can maintain w/ the prescribed limitations.
14. remember algebra????????