Write an algebraic expression for 27 decreased by a number

Y=27-x will be your answer y represents what the outcome will be and x represents the number 27 is being decreased by

All coplanar points equidistant from a given point.

 To understand this definition you must first know that they are coplanar points.
 In geometry, a set of points in space is coplanar if all points are in the same plane.
 We have then that the coplanar points equidistant (same distance) of a given point (center of the circle), forms a circle.

 
 all coplanar points equidistant from a given point.