Family of circles touching the line 4x+3y−12=0 at point (3,0) is (x−3)2+(y−0)2+λ(4x+3y−12)=0
If it passes through the point (−1,−3) then (−1−3)2+(−3−0)2+λ(4(−1)+3(−3)−12)=0 ⇒λ=1
Hence, equation of circle is (x−3)2+(y−0)2+(4x+3y−12)=0
or x2+y2−2x+3y−3=0.