1) Eliminate X or Y by figuring out what you need to multiply one of the equations with to eliminate one of those variables when you add the equations together. So for example, to get rid of the Y variable, multiply the bottom equation by 2:
2(9x-4y=21) ---> 18x - 8y = 42
2)...

The midpoint formula is: [(x1 + x2)/2 , (y1 + y2)/2)]
A data point is always given in the form of (x, y). In your question, 12, 12 would be the data point at x = 12 and y = 12 as well as x = 2 and y = 12. These are 2 different points on a graph.
To solve your equation by using...