Hi,

I have a circle with center (0,0) of radius 4
It's equation should be:
X^2 + Y^2 = 16

I also have a straight line which has gradient of 1 (45deg) which goes through (4,0) and (0,-4)
It's equation should be:
Y = X - 4

I am trying to use calculations to confirm that the line intersects with the circle at points (4,0) and (0,-4) but can only get (0,4). I forget how to calculate the other coordinate (0,-4).

This is how I calculated my first intersect.

Put Y=X-4 value into circle:
X^2 + (X-4)^2 = 16

Simplify down: (Is this part correct?)
2X^2 - 16 = 16
2X^2 = 32
X^2=16
X=4

Now if i use 4 as value X and put back into circle equation then I get 0:
4^2 + Y^2 = 16
16 + Y^2 = 16
Y^2 = 16-16 = 0

So coordinate is (4,0).

Now how do i calculate so i get the other intercept, (-4,0)?