Answer:
Consider the expression
. q is called the quotient, a is is the dividend and b is the divisor.
Since q is a multiple of 6, then q has the form
for some integer k.
Since a is a multiple of 9, then a has the form
for some integer s.
Since b is a factor of 12, then if 12 can be expressed of the form
, for c an integer. Then b has the form 
Replacing the preview expression in the initial expression we obtain:

Then
is a equation to Isabel's problem.