If (x + 3) (x + m) = the square of X + kx-15, then the value of m-k is

If (x + 3) (x + m) = the square of X + kx-15, then the value of m-k is

(x+3)(x+m)
=x²+3x+mx+3m
=x²+(3+m)x+3m
=x²+kx-15
So 3 + M = K
3m=-15
m=-5
k-3+m=-2
So m-k = - 3