Using distance formula, the distance^2 from a,b to each of the points on the circle is the same ....
    to 2,0 and 7,1 :
           (a-2)^2 + (b-0)^2  = (a-7)^2 + (b-1)^2       simplify to 
10a+2b = 46
 
    to  2,0 and 3,5 
           ( a-2)^2  + ( b-0)^2  = (a-3)^2 + (b-5)^2    simplify to 
2a +10b = 30 
    Now you have two equations relating two unknowns....solve for a and b 
Multiply top equation by -5 to get 
-50a - 10 b = -230    and add it to the second equation to get 
-48a = -200 
a = 4.167.      use this value of 'a' in one of the equatins to find  b = 2.167