The given information is:

[You] purchased twice as many shares as

The best way to translate this into an equation is to reword the given information in such a way as it contains the phrases for the relevant unknowns: "the number of shares of", and "the number of shares of".

If you rewrite the sentence using these phrases, you get:

The number of shares of was twice the number of shares of

In symbols, this is:

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