Let's say that Robbie's age is represented by r and Sheila's age is represented by s. Since we know that r is greater than s, we can set up a difference equation of r - s = 12. We also know that r + s = 42. Now, we can simply add the two equations and get that 2r = 54, then finally that r = 54. Therefore, Robbie is 54 years old.