That's quite simple
x is center of circle and ab is diameter....hence x bisects AB
Now there are 2 ways to do this...
Take the coordinate of B as (x,y) and use AX = BX in distance formula and equate or you could do section formula for midpoint

If u do everything right u should get B = (1,-11)
A more detailed explanatuon would be,
 We know that the centre of a circle divides the diameter in half.
Now we take the other pt on the diameter as (a,b)
so, by the distance formula,
from here we get a = 1
and b = -11
hope that it was helpful, if it was pls do let me know