Lists

reseni = Solve[(x^3) + 3*(x^2) - 2 == 0, x]

Part[reseni, 2]

reseni[[2]]

Replacement rules

?x

dosazeni = reseni[[1]]

x = x /. dosazeni

?x

?/.

(a + b)*(2*a - 4) /. {a -> (χ/y), 
        b -> (y/χ)}