Ok so your previous triangle’s shortest side was 3 cm. Now it’s 13.
Just do 13/3 to get 4 1/3.
Then use that to multiply 6 and 9.
4 1/3 x 6 = 24 + 2 = 26
4 1/3 x 9 = 36 + 3 = 39
So your new sides: 13, 26, 39.
And I also just realized 3 is half of 6 and a third of 9. So basically 13 x 2 and then 13 x 3. That gets you the same result, dw :)