What is the correlation or covariance with the market portfolio?

So I'm talking about problem ii. in the photo. I think the beta of stock A is equal to the covariance of stock A with the market, divided by the standard deviation A x standard deviation of the market. Am I right to assume that I need to calculate the covariance of stock A with the market by using the correlation of stock A with the market? Is that correlation equal to the weight of stock A in the market portfolio (25%) or do I need to do something with the correlation of A with B and C to calculate correlation of A with the market? Thanks in advance

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