What does it mean when the exam paper asks me to "assess the validity of X"?

To "assess" the viewpoint which the exam paper has given to you, you must do two things - first you must discuss how the viewpoint is valid and seems correct according to what you have learnt, and then you must also discuss how it could be flawed or perhaps biased. For both of these 'positive' and 'negative' assessments of the viewpoint, you must back-up  what you are saying using facts and examples you have learnt. So for instance, to show the validity of the statement "‘Attempts to recover from the disaster of 1187 failed because of the disunity of the crusaders", firstly you should give examples of how the crusaders were not united, such as the personal rivalries between the crusade leaders Richard I and Philip II, and how this affected the crusaders' success and show the validity of the statement. Then you should assess the other side of the argument to give examples of how there were other reasons for the failure of the crusaders, such as the lack of support from Byzantium, which shows that the statement is not fully valid. Finally, you should decide if you yourself believe if the viewpoint is valid according to which argument you believe to be the strongest, and why. In doing this, you have focused on the statement in the question, but you have also displayed your own broader knowledge, which will achieve the highest marks for you.