Equate both equations to get the solutions. Since its a tangent, it only has one solution.

With the quadratic equation you get:
0= x^2 - 2k^2x + k^4

You can either use perfect square to find the solution, or use b^2 - 4ac = 0.

If you use b^2 - 4ac =0, you get
4k^4 - 4(1)(4k^4) = 0.

Since this equation is true regardless of any value of k, k is from negative infinity to positive infinity

Hope it helps!