First let's reduce the feet to miles
there are 5280 feet in a mile therefore
26400 feet=5 miles
31680 feet=6 miles
Jet A(the first jet) descends 5 miles in 96 miles
Jet B(the second jet) descends 6 miles in 120 miles
We can compare these as fractions to see which is steeper. This can be viewed as slope and the origin (0,0) is the airport.
slope: 6/120=?=5/96
1/20=?=5/96
Now we know that 5/100 =1/20 so 5/96 must be bigger than 5/100 because you are dividing by a smaller number.
so 1/20<5/96
So Jet B is descending steeper than Jet A.
As for linear model, I don't exactly know what your teacher means but I think I actually used the linear model when I'm thinking of steepness as slope in the coordinate plane, I will include a picture.
In this extremely zoomed out graph, you can see the blue line is just slighly higher than the red line(slope as in explanation is way easier to tell) this could be seen as the linear model) :) Hope it helped!
but it is not divisible by 4, because 
but 
.