Desired circle has radius of 40.8 and is tangent to the sloping line. That means the center of the desired circle will lie on a line offset from the sloping line by the radius, 40.8.
Desired circle has radius of 40.8 and passes through a point. That means the center of the desired circle will lie on a construction circle with center on the point and radius of 40.8.
Center of the desired circle will be at the intersection of the offset line and construction circle.