Answer
$\theta =90 ^{\circ}$
Work Step by Step
We know the following:
$\cos\theta=\frac{u\cdot v}{|u||v|}$
Therefore, it follows:
$u\cdot v=4(1)+(3)(-1)+(-1)(1)=0$
Whenever the dot product is zero, the angle between the vectors is a right angle. Thus:
$\theta =90 ^{\circ}$