The problem is over-defined, namely there are three conditions, but only two unknowns. It is possible that we don't have a solution if the conditions do not agree. We'll see.
Use the first two conditions to solve for x, y such that x+y=3.9, x-y=0.9, which gives x=2.4, y=1.5.
Now, multiply 2.4 by 1.5. If the product is 3.6, the solution is fine, if not, the problem is inconsistent.
Can you decide to which case the problem belongs?