Words to the wise: Please express exponentials as x^2, not x2.
x2 + y2 = 81 should be x^2 + y^2 = 81 = 9^2
Comparing this to x^2 + y^2 = r^2, we see that r= radius=9. y-intercepts are where x=0, so purposely set x=0 and solve for y:
0^2 + y^2 = 9^2. Then y = 9 or y = -9.
The graph intersects the y-axis at (0,9) and (0, -9).
