We have two points on a linear scale: (500,100) and (680,127).
From these we can find the slope of the linear relationship:
(127-100)/(680-500)=27/180=3/20.
Now we can plug in either point to find the linear equation:
B-100=(3/20)(A-500)=3A/20-75, B=3A/20+25; or:
B-127=(3/20)(A-680)=3A/20-102, B=3A/20+25 or B=(3A+500)/20.
This can be written in different ways:
20B-3A=500, 3A-20B+500=0, etc.