(mx-3)(nx+2)=6x^2+kx-6 we work on the first side and we get mnx^2+2mx-3nx-6
so it is 6x^2 = mnx^2 --->> mn=6 then 2mx-3nx =(2m-3n)x = kx then 2m-3n=k
we know that m+n=5 since we know that mn=6 and m+n=5 m and n are the roots of the equation
x^2-5x+6 solving this equation we have the roots, they are 3 and 2 so it will be m=3 and n=2
or m=2 and n=3 substituting these values to the equation 2m-3n = k we get k=-5 and k=0