Matrices to solve a vector combination problem.

So I’m just starting out learning this at home.

Lets say we have two vectors

a=[3/-6] <6 below the 3 b=[2/6] again the 6 is below.

a+b=c =[7/6]

Then the book goes off into saying

ax+by=c but in matrix form, there’s a graph of a, b and c next to it and I know x in ax is suppose to be the x coordinates and y is the y coordinates, but what x and y coordinates?

[3/-6]x+[2/6]y=[7/6]

[3/-6]x+[2/6]y= [3, 2/-6, 6][x/y]

the book says 3 x X = 7 but when did we work out what x was and the same with y