sound linke yu want strate line thru 2 points: (1,4) & (3,0)
deltax=3-1=3
deltay=0-4=-4
slope=deltay/deltax=-4/3
line: y=(-4/3)x + konst
projekt line from x=1 tu x=0...deltax=-1, so deltay=slope*deltax=(-1)*(-4/3)=+4/3
y=4-4/3=(12/3) -(4/3)=-8/3
line formula: y=(-4/3)x -8/3
###### me assume yu want y at x=6...deltax=6, deltay=(-4/3)*6=-24/3
y at x=6...(-4/3)*6 -8/3=-(24/3) -(8/3) =-32/3
or -10.6666666666666...