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Math·Advanced Math·Nonlinear equations in one variable and systems of equations in two variables

hard

$\frac{5}{9}(5x+9)\left(x+\sqrt{5k+9}\right)\left(x-\sqrt{5k+9}\right)=0$
In the given equation, $k$ is a positive constant. The product of the solutions to the equation is $72$. What is the value of $k$?

1

31/5

2

6.2

3

6.20