One root is already 0, because of the x outside the parentheses.
Now you have to find 2 more roots with the equation x^2+7x+k that you found.
Find a value of k that will work.
So what two numbers will multiply to k and add to 7.
That can be 12, since 3 x 4=12 and 3+4=7.
x(x+3)(x+4)
That will give you 3 real solutions.
k=12