one may note, you kinda missed including the related pictures.
so for every added bin, the stack grows by 4 inches. rise/run, the run is 1, the rise is 4, 4/1 = m = slope
now.... in the picture not shown hehee, the 10-gallon bins, 4 of them measure up to 29 inches high.... wait a second.. I thought the rise was only 4?
well, there are 4 bins, like in this case, so the rise should be 4*4, or 16, but there are 29 inches, that means, the slack leftover is the "initial value", or y-intercept, which should be 29 - 16, or 13.
[tex]\bf slope=m=\cfrac{rise}{run}\implies \cfrac{4}{1}\implies 4
\\\\\\
y-intercept\implies 13
\\\\\\
y=mx+b\implies y=4x+13[/tex]