829 is rounded to 830 because the 9 is greater than 4 so its going to be rounded up.
The two triangles with the given perimeter is attached .
Perimeter is the sum of all sides.
For the first triangle , sies are 2a,2a and b.
So the perimeter is

For the second triangle, sides are a,a, and 2a+b. Therefore perimeter is

So for both triangles, perimeter is

Your answer would be
10 - k