Question 1202498
The "graph below" might help.


Your description gives something like this:
(x,y) points for x gallons of fuel, y weight of plane in pounds
y=mx+b, for m=5.7; and given point (54, 2207.8).


Directly using the point-slope equation form for a line,
{{{y-2207.8=5.7(x-54)}}}.
Do what you want or need from that.