Considering the vertex of the quadratic equation, it is found that the scuba diver reaches maximum depth after 6 seconds.
<h3>What is the vertex of a quadratic equation?</h3>
A quadratic equation is modeled by:

The vertex is given by:

In which:


Considering the coefficient a, we have that:
- If a < 0, the vertex is a maximum point.
- If a > 0, the vertex is a minimum point.
In this problem, the depth is given by:
D(x) = -x² + 12x + 4.
Which means that the coefficients are a = -1, b = 12, c = 4, hence:

The scuba diver reaches maximum depth after 6 seconds.
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