Recall the relationship between image distance (v), magnification (m), and focal length (f) for a convex lens.

Here's how to solve the problem:

a) (i) Use the graph to find a value for the focal length of the lens.

Step 1: Recall the relationship between image distance ($v$), magnification ($m$), and focal length ($f$) for a convex lens.
The lens formula is $\frac{1}{f} = \frac{1}{u} + \frac{1}{v}$, where $u$ is the object distance.
The magnification is $m = \frac{v}{u}$. From this, we can express $u$ as $u = \frac{v}{m}$.
Substitute $u$ into the lens formula:

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