manipulating symbolic variables using "solve"

hi,

i've written a system of equation (non linear) and i solve it using "solve"

for example:

v(1) = x1+y2
v(2) = z4-3*x1*y1
..

..now, i have written another system of equations with the same variables and i would like to use the solutions (values founded by the function) in it..

w(1) = x1-x2 ..

where x1 for example is now the value founded before for the symbolic variable x1..

i need to do this because the second is a system of disequations so i can't simply put them together in the previous system (they are all disequations and i need to check that they are <0)..

how can i do? i'm thinking about it but i need some suggestions :)

thank you