Find the equation of the circle that passes through the origin and has its center at (-3,-4). The equation of a circle with center (h,k) and radius r is (x - h)² + (y - k)² = r² We do not know the radius r, but we do know h and k, (h, k) = (-3, -4), so we have (x + 3)² + (y + 4)² = r² Now since we are told that the circle passes through the origin, then we can substitute (x, y) = (0, 0) (x + 3)² + (y + 4)² = r² (0 + 3)² + (0 + 4)² = r² 3² + 4² = r² 9 + 16 = r² 25 = r² 5 = r So the equation is (x + 3)² + (y + 4)² = 5² (x + 3)² + (y + 4)² = 25 Your teacher may want you to multiply that out and get x² + y² + 6x + 8y = 0Edwin