Use the character in the expression to obtain the coordinate of the last point, as shown in the following example:
Command: line
Specify first point: 'cal
>> Expression: cen+[0,1]
>> Select entity for CEN snap: Select a circle or an arc
Specify next point or [Close/Undo]: 'cal
>> Expression: +3*vec1(cen,cen)
The first point of the line is 1 unit in the Y direction from the center of the first selected circle. The second point of the line is 3 units away from the first point. The direction of the line is from the center of the first selected circle to the center of the second selected circle.