A point (x,y) is 5 units from the origin means that:

x^2 + y^2 = 25

which is the required equation of a circle centred at (0,0) of radius 5 units.

A point is 4 units from (2, -1) means that:an equation of the locus of points 4 units from the point (2,-1) is

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

which is the required equation of a circle centred at (2,-1) of radius 4 units.

RonL