You didnt say how many words but...............
I suggest you firstly define what is meant by organisational culture and why it brings about success in an organisation, Give a real example or two.
Then describe how for a takeover to be successful, there is an idea about how the organisations can compliment one another but for this to work they have to be able to work together, value the same things, approach things in similar ways. This is usually underestimated. Again, think of an example.
Then google a couple of takeovers, work out whether you think they have been successful or not (and why), and attempt to explain this in terms of culture. You may be able to, you may not - the question says "to what extent".
Then draw a conclusion as to whether you think culture is (or is not) a significnat issue in the success of takeovers.