You need to find the points of intersection of the two circles. The circle with center at has radius , the circle with center at has radius . Since supplier 1 charges $20 less than supplier 2, describes the relationship between their radii such that the total cost is the same from each.

The equations of the two circles are:

Combine the two equations, and find the radical line to describe the set of points you're looking for.

Circle-Circle Intersection -- from Wolfram MathWorld

Some of the points you will find (if I did my math right):

at

at

at

at