A scale factor "from A to B" is a number that multiplies some dimension of A to make it equal to the corresponding dimension of B. That is, ...
(scale factor A to B) = (dimension of B)/(corresponding dimension of A)
Your triangles have one corresponding dimension. On triangle A, it is 6 ft. On triangle B, it is 5 ft. Using the above formula, we have
(a) (scale factor A to B) = (5 ft)/(6 ft) = 5/6
The reverse is also true. That is, ...
(b) (scale factor B to A) = (dimension of A)/(corresponding dimension of B)
(scale factor B to A) = (6 ft)/(5 ft) = 6/5
(c) The scale factors are reciprocals of each other.