Question

find the standard equation of a circle center is (1,3) and tangent to line 5x-12y-8=0.

Answer 1

Hint : Distance between center and tangent = radius

Answer 2

i have the answer (x+1)^2+(y-3)^2 =9

but in the answer book they have an intermediate step that looks like this and
i have no clue so i thought i would post here:

|5(1)- 12(3) -8| / 13 = 3

i understand the numerator on the left, but where did the denominator or the right part
come from.