Question 600915
You cannot actually solve this equation for y - all you can do is express y using the other terms

Subtracting 7x from both sides of the equation we have 


7x-3y-7x = 4-7x
i.e. -3y = 4-7x
i.e.  -y = (4-7x)/3
i.e.   y = (7x-4)/3 which is how you could express y using the other terms.


If you need further clarification on what is happening here you may email sumanapala@gmail.com.