Question 1087626
Let 
F be the father's age
C be the child's age

Suppose,
F= 5x-3

and 

C=x

For instance, the child is 15. By substitution, F= 5(15)-3. We then simplify which result to 72, the age of the father.

Ok let's check
The father is 5 times 15, the age of the son, subtracted by 3 since 3 years younger. Hence, we get 75 minus 3 which result to 72. Therefore, F= 5x-3 is the
equation for x values permitted.