When we do a data sufficiency question, it is best to start by asking ourselves: "What do we need?" In this case, we need to know what xy is. So we either need an equation or a system of equations that provides us with one single value for both x and y or that lets us eliminate parts and come down to xy, which was already in the equation.

In this case, statement 1 doesn't give us xy nor does statement 2. However, together, we have a system of equations that will let us solve for the exact values of x and y.

It may seem like a waste of time to think at the outset of the possible ways that the GMAT test writers will give us (or not) the information, but it helps us to be more proactive and less reactive to the data that are presented.