This is a very standard problem. Do you have a textbook? It should have examples...
I'd be surprised if you were asked to solve this without first being given the proper tools.
In the case of the question posted, there is a unique solution. However, when there are more equations then unknowns it is still possible for equations to be:
1. Inconsistent (in which case there is no solution). eg. x + y = 3, 2x + 2y = -1, x - y = 2 (three equations, two unknowns, no solution)
2. Redundant (in which case there may be infinite solutions). eg. x + y = 3, 2x + 2y = 6, -x - y = -3 (three equations, two unknowns, infinite solutions).