to make equation of line through two points :
now your two points
so you will have
if you don't trust me just put any x from 4 to 10 and you will get exact value of y...
for example x=7 -> y=0 (as you see on graphic)
if u need to represent your function over step functions it would be like this
P.S. you can take any two points from that line and put in formula and you will get same result (doesn't have to be "start" and "end")