Answer:
The vertex is the point

Explanation:
we know that
The general form of an absolute value equation is

The variable a tells us how far the graph stretches vertically, and whether the graph opens up or down. The variables h, and k, tell us how far the graph shifts horizontally and vertically
The vertex is the point (h,k)
we have

Rewrite the expression (3x+1)

substitute



therefore
The vertex is the point

see the attached figure to better understand the problem