Question

What is the equation of a circle with center (1,2) and radius 3?

Answer 1

put it in the explicit form:
\[(x- \alpha )^{2} + (y- \beta ) ^{2}=r ^{2}\]
Alpha and beta are the coordinates of the center of the equation, and r is the radius of the circle.
Just plug the ones that you are given and you will have the desired equation of a circle.