This is how I do it but it might sound a bit confusing ....
You basically look at your venn diagram and tackle the given that part first 'given that she is left handed and female'
and see how many people there are that wear glasses and are female... in your diagram, it's a total of 4.
This becomes the denominator.
Then in this overlap region, you look for the bit where the glasses circle overlaps with female and Left handed or in other words, the number of people who are LF, wear glasses and are female which is 1 here. This part becomes your numerator.
So 1/4 should be your answer...
Hope that helps!