Alright, the large garden is garden 1. The small one is garden 2. means a side of the first garden...

You know that:

And that:

Thus:

Then:

Now go back to this one:

Substitute:

Then:

Now spread out the negative sign:

Group like terms:

Subtract:

Add 156.25 to both sides:

Divide by 25:

So the greatest length can have is 10.25. But check my arithmetic!!!