Find the probability of tossing 3 tails, then 4 heads, on the first 7 tosses of a fair coin

The general form of the quetion of a circle is ax^2+by^2+cx+dy+e=0 where a=b=0 if the circle has a radius of three units and the center lies on the y axis which set of values of a, b, c, d, and e might correspond to the circle