[tex] \text{The formula of a circle with a center (h; k) and a radius r}\\\\(x-h)^2+(y-k)^2=r^2\\\\x^2+y^2=4\to (x-0)^2+(y-0)^2=2^2\\\\\text{therefore we have the circle with a center (0; 0) and a radius 2}\\\\x-y=1\ \ \ |-x\\-y=1-x\ \ \ |\cdot(-1)\\y=x-1\\\\for\ x=0\to y=0-1=-1\to(0;-1)\\for\ x=1\to y=1-1=0\to(1;\ 0)\\\\\text{Look at the picture} [/tex]
