Answer:
k = 2a = 10
Step-by-step explanation:
If
x² / 25 − y² / 56 = 1
we know that
a² = 25  ⇒  a = 5
b² = 256  ⇒  b = √256
The equation of the  hyperbola defined by |FP − PG| = k can be written as follows:
|FP − PG| = 2*a
then
2*a = 2*5 = 10