```
{a, b} = {6, 7};
t = 0;
dt = 0.001;
Do[{l, m} = With[{r = NSolve[{x - y == a, 2 x + y == b}, {x, y}]},
    Extract[r, Position[r, _?NumericQ]]];
    t = t + dt;
    a = a + (l + m)a dt;
    b = b + l m b dt, {1000}]

```

How to plot `a(b)`, `a(t)` ?