In two weeks three cows eat all the grass on two hectares of land, together with all the grass that regrows there during the two weeks. In four weeks, two cows eat all the grass on two hectares of land, together with all the grass that regrows there during the four weeks.
 How many cows will eat all the grass on six hectares of land in six weeks, together  with all the grass that regrows there over the six weeks?

 (Assume: 
$\bullet$ the quantity of grass on each hectare is the same when the cows begin to  graze,
$\bullet$ the rate of growth of the grass is uniform during the time of grazing,
$\bullet$ the cows eat the same amount of grass each week.)
1. Let \( x \) be the amount of hectares that a cow eats per week, and \( y \) be the amount of hectares of grass that grows per week.

2. From the problem, we have two scenarios:
   - In two weeks, three cows eat all the grass on two hectares of land, together with all the grass that regrows there during the two weeks.
   - In four weeks, two cows eat all the grass on two hectares of land, together with all the grass that regrows there during the four weeks.

3. We can set up the following equations based on the given information:
   - For the first scenario: 
     \[
     2 \cdot 3x - 2 \cdot y = 2 \quad \Rightarrow \quad 6x - 2y = 2 \quad \Rightarrow \quad 3x - y = 1
     \]
   - For the second scenario:
     \[
     4 \cdot 2x - 4 \cdot y = 2 \quad \Rightarrow \quad 8x - 4y = 2 \quad \Rightarrow \quad 4x - 2y = 1
     \]

4. We now have the system of equations:
   \[
   \begin{cases}
   3x - y = 1 \\
   4x - 2y = 1
   \end{cases}
   \]

5. To solve this system, we can multiply the first equation by 2 to align the coefficients of \( y \):
   \[
   2(3x - y) = 2 \quad \Rightarrow \quad 6x - 2y = 2
   \]

6. Now we have:
   \[
   \begin{cases}
   6x - 2y = 2 \\
   4x - 2y = 1
   \end{cases}
   \]

7. Subtract the second equation from the first:
   \[
   (6x - 2y) - (4x - 2y) = 2 - 1 \quad \Rightarrow \quad 2x = 1 \quad \Rightarrow \quad x = \frac{1}{2}
   \]

8. Substitute \( x = \frac{1}{2} \) back into the first equation:
   \[
   3\left(\frac{1}{2}\right) - y = 1 \quad \Rightarrow \quad \frac{3}{2} - y = 1 \quad \Rightarrow \quad y = \frac{1}{2}
   \]

9. Now we know \( x = \frac{1}{2} \) and \( y = \frac{1}{2} \).

10. To find how many cows will eat all the grass on six hectares of land in six weeks, we set up the equation:
    \[
    6 = \frac{1}{2} \cdot 6 \cdot \text{number of cows} - \frac{1}{2} \cdot 6
    \]

11. Simplify the equation:
    \[
    6 = 3 \cdot \text{number of cows} - 3
    \]

12. Add 3 to both sides:
    \[
    9 = 3 \cdot \text{number of cows}
    \]

13. Divide by 3:
    \[
    \text{number of cows} = 3
    \]

The final answer is \(\boxed{3}\) cows.