In planning an answer to this question, it is important to first deconstruct the question itself. By doing this, you can gauge what it seems the examiner is looking for, and plan your response accordingly.'EXTENT' - The first key point is that of the question asking to you assess the 'extent' of something. When a question is asking about 'extent', it is essentially asking 'how much impact'. This allows you to write a nuanced response, where, at every stage of your answer, you come back to a judgement on just how far Johnson's presidency changed the course of the movement, in comparison to other events and/or actions.FACTORS OR THEMES - This question could be answered by paragraphs analysing different 'factors' (e.g. A paragraph on Lyndon's presidency, a paragraph on the work of individual activists such as Malcolm X, a paragraph on Roosevelt's 'New Deal'). However, choosing to write thematically (e.g. A paragraph on political turning points, a paragraph on economic turning points, a paragraph on social turning points) may show a greater complexity and depth of response. Thematic answers are also a good approach, as they allow you to easily refer to the turning point mentioned in the question (Lyndon's presidency) at every stage of your answer, which is crucial. Also, they allow you to compare and contrast a wider range of potential 'turning points', which is important in your analysis of 'extent'.TURNING POINT - The phrase turning point means the question is looking for analysis of events/actions that changed the course of the Civil Rights Movement in America. What used to pose a stumbling block to me with this phrase was the idea that 'turning points' don't always have to be a change for the better. Once I had realised this, it really helped the level of analysis in my answers, as I could incorporate instances of significant negative progress of the Movement into my answers. For example, in this question you say that, for some, Reagan's presidency may have been an economically negative turning point, due to his apparent commitment to cutting back on affirmative action. You could then evaluate this point by highlighting the fact that Reagan's impact was limited by a liberal congress.
Throughout your answer, use relevant and specific examples to make your point. However, don't feel overwhelmed by dates and statistics! Having three or four points per paragraph is enough to write a strong argument, and you won't be expected to have memorised hundreds of examples for each question. In fact, 'turning point' questions lend themselves well to having a handful of significant events learnt, as not every single date or event or person can be cited as a 'turning point' of the Civil Rights Movement in the first place.