Economicus,Originally Posted by economicus
Perhaps these examples will help.
I find no pleasure in washing cars.In those both in and at roughly = in/at the act of, and by = through the act of/by means of.
Are you good at washing cars?
Then you can help me by washing my car.
So in your examples I think the first needs by (but I am not sure what you mean in the sentence) and the second needs at.
There are other uses, especially of by.
Post back if that's not clear.