I believe this is what you want : https://www.desmos.com/calculator/tdlapeleht
The circle with its center at the origin, x^2 + y^2 = 1, is reflected across the line x + 2y = 7.
The reflected circle will have its center on the line that is perpendicular to x + 2y = 7 → y = 2x. The intersection of this line and x + 2y = 7 occurs at (1.4, 2.8). So.......the center of the reflected circle will be as distant from this point as the circle at the origin is from this point. And this point is 1.4 units from the right and 2.8 units up from the center of x^2 + y^2 = 1.
So......the reflected circle's center will be 1.4 units from the right and 2.8 units up from (1.4, 2.8)......i.e., the circle will have the equation (x - 2.8)^2 + (y - 5.6)^2 = 1
