For what va

For roots to be real D ≥ 0

⇒ b² - 4ac ≥ 0
For x² + 4x + k = 0, 

⇒ 4² - 4(1)k ≥ 0

⇒16 – 4k ≥ 0

⇒ – 4k ≥ - 16

⇒4k ≤ 16

⇒k ≤ 4

Or
Consider 3x² – 10x + k = 0,
Compare the given equation with the general equation ax² + bx + c = 0
So, a = 3, b = -10, c = k

Let, the roots of the equation be x and 1/x.

Product of the roots

⇒ k = 3