AQA A Level Biology Practice Questions (chapter 1)

This is quite specific but hopefully someone can help me! I’ve just done the practice questions at the end of Chapter 1 in the text book (Oxford university press) and there is a question that even with the mark scheme, I have no idea where the answer comes from. If anyone has the same book and has done the questions and understands I would really like to know how to work out the answer to question 3)b). Thanks!

Edit: I’ve found the same question on revisely (still no idea where the answer comes from) https://www.revisely.com/alevel/biology/aqa/questions/biological-molecules/proteins-and-enzymes/3

To find the rate of a reaction, you need two things: i) the change in the thing you're measuring and ii) the amount of time this change happened over. This would be shown in an equation of rate = change / time. For this example, you want to find how much the concentration (g dm-3) has changed in a certain length of time. You can do this by picking any two points from the linear graph that it gives you, finding the difference, and dividing it by the difference in time. Give it a go by using the concentrations at two chosen time points.

Does that get you there? If not, here's some more detail: