sound like yu wanna make a strate line tween 2 points: (1,5) & (2,10)
deltax=1, deltay=5, slope=deltay/deltax=5
line formula: y=5x+konst
projekt line from x=1 tu x=0...deltax=-1
deltay=slope*deltax=5*(-1)=-5
so y=5-5=0
line formula: y=5x+0, same as y=5x