a) We have to find the equation of a circle with center c = (4,6) and diameter D = 3 ft.
We can express a circle with center (a,b) and radius r as:

Then, in this case, r = D/2 = 1.5, so we can write the equation as:

b) We now have to place another circle with its center 7 ft to the right and 2 ft up from the previous sprinkle.
Its radius, due to the increase in the preasure, will be two times the radius of the previous cicrle. Then, for this circle, r = 1.5*2 = 3 ft.
We can calculate the new center as:

With a center of (11, 8) and radius r = 3, the circle can be expressed as:

We can graph both circles as:
Answer:
a) (x - 4)² + (y - 6)² = 2.25
b) (x - 11)² + (y - 8)² = 9