first, the other line go thru 2 points: (-3,1) & (5,-1) . . . deltax-5-(-3)=8, deltay=(-1)-1=-2 . . . slope=deltay/deltax=-2/8=-1/4 =-0.25 . . . line yu want=90 degrees from this, so it hav slope=+4 . . . y=4x+konst . . . line go thru (3,4) . . . projekt line from x=3 tu x=0...deltax=-3 . . . deltay=slope*deltax=4*(-3)=-12 . . . y=4-12=-8 . . . y=4x-8