So, we know that a^2 + b^2 + c^2 = 13.

Also, ab + bc + ca = 6.

And we want to find a + b + c. So, we sqaure a + b + c.

That would be a^2 + ab + ac + ba + b^2 + bc + ca + cb + c^2.

We group the squares together to get a^2 + b^2 + c^2, which is 13.

Then we group the ab or bc or ac together.

That gets us 2(ab + bc + ac) which is 2*6 which is 12.

Then we add to get 13 + 12 = 25.