Chapter 1 - Section 1.9 - Distance and Midpoint Formulas; Circles - Exercise Set - Page 280: 34

$(x-2)^2+(y+1)^2=16$

RECALL: The standard equation of a circle whose center is at $(h, k)$ and has a radius of $r$ units is: $(x-h)^2 + (y-k)^2 = r^2$ The given circle has its center at $(2, -1)$ and a radius of 4 units. Thus, the standard form of the circle's equation is: $(x-2)^2+(y-(-1))^2=4^2 \\(x-2)^2+(y+1)^2=16$