Let the unit vector in the direction of \(\vec A=3\hat i+4\hat j-12\hat k\)
So any unit vector in the direction of \(\vec A=3\hat i+4\hat j-12\hat k\)
\(\vec B=\cfrac{\vec A}{|\vec A|}\)
So the magnitude of the vector \(|\vec A|=\sqrt{3^2+4^2+(-12)^2}\)

So, the unit vector
\(\hat B=\cfrac{3\hat i+4\hat j-12\hat k}{13}\)