Since x = -3 is the only value that makes the quadratic = 0.....then this has a "double" root

So...the sum of the roots = -3 + -3 = -6

And the sum of the roots in the form ax^2 + bx + c = -b/a

So...in our case, the sum of the roots are = - m / 1 = -m

So

-m = -6

m = 6