Question 40689
Hmmm...let's see if I've got this right:

The roof is 7 meters from peak to eave.  This forms one side (the hypotenuse) of a right triangle, right?
The attic space is 3 meters high at the peak.  This forms the perpendicular height of the same of the same right triangle.
So you need to find the distance, x, of the base of this triangle, and this goes halfway across the attic.

Calling upon Pythagorus, you have:
{{{7^2 = x^2 + 3^2}}}
{{{49 = x^2 + 9}}} Subtracting 9 from both sides:
{{{x^2 = 40}}} Taking the square root of both sides, you'll get:
{{{x = 6.3}}} meters. 

Now since this is halfway across the attic, you'll need to double this to get 12.65 meters (Approximtely)