Slope is equal to the change in y over change in x, or rose over run m= 1/3
Based on the picture, PC = PD
Unit vector along the direction v = <3,1,-4> is :

So, unit vector opposing the
is :

so, vector of magnitude 3 units in opposite direction from v is :

Hence, this is the required solution.
I hope the picture is legible.
The answer is 960 ÷ 4 = 240. I hope this helps