Vickers hardness: Distance between indentations

I'm writing here because I don't know how to solve this question that appeared on an exam (undergraduate, mechanical engineering).

The Vickers hardness test was done on a steel plate twice, once with a smaller force and once with a larger force. The first average length of the diagonal is d=0.181 mm. The second indent (larger force was used this time) left the average length of the diagonal d=0.572 mm. What is the minimum distance between indentations?

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