You have to find the slope of the line. It will give you the conversion rate from inches to centimetres or the other way around.
Looking closely at the graph, or using a ruler as reference, you can estimate that 30 cm ≈ 12 in. In terms of the graph, the line very nearly passes through the point (12, 30). It also passes through the origin, (0, 0), since 0 cm = 0 in exactly.
The slope of the line is then
which means that 1 in ≈ 2.5 cm.
So, if Sarah's height is 64 in, we convert this to cm and find her height is (approximately)