b = 36.0
Explanation:
These two triangles are similar, so they can have a ratio that is equal to one another. We need to pick a side on one of the triangles and match the corresponding side on the other triangle, an example of this would be 6/27. Something like 6/45 would be incorrect since they are totally different corresponding sides. For the other side of the ratio, you will want to keep the same pattern. So if you do the left triangle first in the first ratio, keep it the same for the other. An example of this would be 6/27 = 8/b. It would be incorrect to put 6/27 = b/8.
There are numerous different ratios you can do but this is the one I did...
Notice how I used both of the triangles corresponding sides and followed the pattern of which triangle came first.
Now we can solve by cross multiplying.
Divide.
b = 36