You can add numbers mentally if there are zero's behind them.
First, recall that
So if 
, then 
.
Second,
We know that 
and 
, which means we should also have 
.
Third,
but as we've already shown, we need to have , so we pick the negative root.
Finally,
Unfortunately, none of the given answers match, so perhaps I've misunderstood one of the given conditions... In any case, this answer should tell you everything you need to know to find the right solution from the given options.
I think that using the elimination method is the best, because it is the only method that I use when solving these types of equation. For example, for these two equations, you need to multiply one of the equations by a certain number so that you'd be able to find x and y. The elimination method is the method that tells you to do that. It is also the easiest method.
~Hope I helped!~
