Question 1210263
You wrote that wrong.  Say it like this:

A water container "weighs" 5 kg when filled full with water.  When the container is half empty, it holds 2.6 kg of water.  



x, amount filled
y, how much kilograms in container


Two points are   (0.5, 2.6) and (1, 5).


Slope would be {{{m=(5-2.6)/(1-0.5)=4.8}}}.


Point-Slope equation format could be used.
{{{y-5=4.8(x-1)}}}

{{{y-5=4.8x-4.8}}}

{{{y=4.8x-4.8+5}}}

{{{y=4.8x+0.2}}}
So if container were empty, x=0, so y=0.2.
The container is  0.2 kg when empty.