Question

what is the value of k ?

Answer 1

${x}^{2} - 4x + k = 0 \\ {x}^{2} + kx - 4 = 0 \\ k = - 4$

Answer 2

substitute the equation
x^2-4x+k-x^2-kx+4
=-x(k+4)+1(k+4)
(1-x)(k+4)
x=1
k=-4