In this case, the vertex is the lowest point of the V shaped curve.
The goal is to get the stuff inside the absolute value (the 2x-5) to be as small as possible, so we can get to the lowest point. In terms of absolute value, we want the 2x-5 to be equal to 0
This allows us to set up the equation below and solve for x.
2x-5 = 0
2x = 5
x = 5/2
x = (4+1)/2
x = 4/2 + 1/2
x = 2 + 1/2
x = 2 & 1/2
Let me know if this clears things up or not.