In order to show that these triangles are similar given the side lengths, you have to prove the lengths are proportional.
The side lengths can be written as
3:6 or 3/6. The bases can be written 5:8 or 5/8.
Set up proportion -
3/6 = 5/8
Clearly they are not proportional because 3/6 is .5 and 5/8 is .625
Had Triangle B’s base been 10, the two triangles would have been similar with a scale factor of 2.
Hope this helps!