An easy way to remember is to change the sign of the numbers h and k as described in the previous response. If they are negative, make them positive, if they are positive, make them negative. Remember to write this as a coordinate.
So, the center would be (-3, 1)
Also, to find the radius find the square root of the number the equation is set equal to.
So, the radius would be 4.
A circle with center (h, k) and radius has an equation (x - h)^2 + (y - k)^2 = r^2.
The circle we are given here is (x+3)^2 + (y-1)^2=16
=> (x - (-3))^2 + (y - 1)^2 = 16
=> (x - (-3))^2 + (y - 1)^2 = 4^2
This gives h = -3, k = 1 and r = 4
The center of the given circle is (-3 , 1) and the radius is 4.