SAT Question 1b2b20b9: Circles answer and explanation with Desmos solution

The equation of a circle in the xy-plane can be written as $(x-t{)}^2+(y-s{)}^2=r^2$, where the center of the circle is $(t,s)$ and the radius of the circle is $r$. It's given that circle A is defined by the equation $x^2+(y-6{)}^2=7$, which can be written as $(x-0{)}^2+(y-6{)}^2={\left(\sqrt{7}\right)}^2$. It follows that $r=\sqrt{7}$ and the radius of circle A is $\sqrt{7}$. It's also given that circle B has the same radius as circle A. If the equation of circle B is $(x-h{)}^2+(y-k{)}^2=a$, then $a=r^2$. Substituting $\sqrt{7}$ for $r$ in this equation yields $a={\left(\sqrt{7}\right)}^2$, or $a=7$. It follows that the value of $4a$ is $4(7)$, or $28$.