More SAT practice problems that are beyond me...

John can complete a job in 1/2 the time it takes Harry to complete the same job. If they both work together they can complete the job in 4 hours. How long does it take John to do the job alone?

Sam can do a job in x hours. Denise can do the same job in x + 2 hours. How long will it take Sam and Denise to do to the job together? (Answer must be expressed in terms of x)

This is what I have so far: (1/x) + (1/2+x) = 1/x

Where do I go from here, assuming that this is correct?