For this case we have the following function:

By definition, we have that a linear equation is of the form 
On the other hand, a quadratic equation is of the form
Then, the given equation is not a linear equation, it is not of the form 
Answer:
No, the equation is not linear. It is not of the form 
You would have 10 muffins left because 1/3 of 15 is 5 so 15-5=10