########### line GO THRU 2 points #############
points=(-3,3) & (1,1)
deltax=1-(-3)=4
deltay=1-3=-2
slope=deltay/deltax=-2/4=-1/2 or -0.5
y=-0.5x+konst
projekt line...x from -3 tu 0...deltax=3
deltay=slope*deltax=-0.5*3=-1.5
y at x=0 is 3-1.5=1.5
formula: y=-0.5x+1.5