She did not simplify under the square root correctly.
<h3>Quadratic equation</h3>
Given the quadratic expression
5x^2 - 6x - 8 = 0
Using the general formula, we will have:

From the given equation;
a = 5
b -6
c = -8
Substitute the values into the formula to have:

From the given solution, we can see that she did not simplify under the square root correctly.
Learn more on general formula here; brainly.com/question/21466038