According to the meaning of the question, the distance from the center N( 1, 3) to the tangent 3x-4y-7 = 0 is equal to the radius r.

Then we can get from the formula of the distance from a point to a straight line:

R=d=|3- 12-7|/ root sign [3? +(-4)? ]= 16/5

So the equation for finding a circle is:

(x- 1)？ +(y-3)？ =256/25