If (X-5) (x + 3) is determined by the square of X - kx-15, then k =?

If (X-5) (x + 3) is determined by the square of X - kx-15, then k =?

Work it out
(x-5)(x+3)
=x(x+3)-5(x+3)
=x^2+3x-5x-15
=x^2-2x-15
k=-2
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