Let's use the variable x to represent the number of lawn Tim mows.
If he earns $120 plus $30 for each lawn, we can write the following equation for his earnings:

Then, we want an earning greater than $310, so we have the following inequality:

Now, to find the earning for 9 lawns, let's use x = 9 and calculate the total earning:

Therefore his earning after mowing 9 lawns is $390.