FindRoot, like many other functions in Mathematica, outputs its results as rules. Please see this article on how to use them:
http://support.wolfram.com/kb/12505
So if you wanted the x value from FindRoot, you'd use:
x/. FindRoot[{f[a, b, x] == 0, g[a, b, x, y] == CantidadX}, {{x, 1}, {CantidadX, 1}}]
