Question 116906
OK, I'll start you off on this, have a go after what I've said and then mail back and let us know where you've got to.

1) The city wall is a circle (says so in problem)

2) If we can find radius of the circle, we can solve the problem

3) We can assume the center of the city(circle) is at coordinates (0,0)

4) The coordinates of the house are (0, r+3)

5) The coordinates of the point we can 'just' see the house from are (-9,-r)

Make sure you agree with points 1 through 5. Now, ask yourself what does it mean in the question when it says 'and it can just be seen'. What does this tell you about line between the house and where we can see it from and the circle? Can you write this something down as an equuation and solve for r?