We want the equation of the line with positive slope that is tangent two the two circles:
If you let be a point on
, then and it slope at x=a is .
Also, let be a point on
, then and its slope at x=b is
These slopes have to be the same, so setting them equal and solving for a we get ..........
Just using the slope formula, the slope of AB is:
This has to be the same as the slope at A:
Sub in  and solve for b and we get:
This means that
This gives a slope of
So, we have two points and a slope. Almost there.
Using these we find we have a line equation of: