These kinds of problems are definitely tricky!

Let the time Ada takes to do a job be x hours.

Then the time Betty takes to do the same job is x+2 hours.

Therefore, Ada can do of the job in 1 hour.

And also, Betty can do of the job in 1 hour.

Working together they can complete of the job in 1 hour.

The question says, that together they work for three hours and betty works alone for 1 hour to complete the job. It follows that:

Now solve for x.