How do you implicitly differentiate #7xy- 3 lny= 42#?
Now take a look at -3lny. Since 3 is a constant in front, you only need to worry about the derivaitve of lny and then multiply the 3 later.
The derivative of a constant is always 0 so the equation will be equal to 0. Now combine your two parts together.
Subtract 7y on both sides.
Then factor out