Answer:
The distance is 
Step-by-step explanation:
From the question we are told that
The speed at which Gabby rode his bike is 
The average speed of the car is 
The distance from Gabby's house to the parents house is 
The time taken for Gabby to get to his parents house is t = 9 hours
Generally the distance from Gabby's house to his parent house is mathematically reparented as

Here
is the distance from Gabby's house to his sisters house which is mathematically represented as

=> Here
is the time taken to move from Gabby's house to his sisters house
while
is the distance from his sisters house to his parents house which is mathematically represented as

So

=> 
Generally the total time taken to arrive his parents house from his house is mathematically represented as

=> 
Solving equation 1 and 2 simultaneously we have that


multiply equation 2 by 18 and equation 1 by 1


Substracting equation 3 from 4

=> 
=> 
substituting
into equation 2

=> 
So the distance from Gabby's house to his sisters house is mathematically represented as

=> 