Determine the slope of the tangent to y=x^2+9x+9 at the point where the curve intersects the line y=3x.

The answer in the textbook is just 3. Why is this so? I don't know how they got this.

The slope of the LINE is 3. But don't I have to find the slope of the tangent? in which case I find the derivative?

I did this:

y'=2x+9

but then how did they get 3?