Subgroup in Free group of rank $2$ is isomorphic to a free group of rank $3$

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Consider the free group of rank $2$, denoted by $\mathbb{F}(x,y)$.

I want to show that the subgroup generated by $\{x^{2},xy,y^{2}\}$ is free group of rank $3$.

How should I approach this problem ? Any hints.

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When dealing with free groups, I like to think about finite graphs. In your situation, notice that you have the following covering of graphs (which is $\pi_1$-injective in particular):

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