You want your deflection in inches, so your length is 300", not 25'.
The way I read it, it would be (6000 x 300^3) / (48 x 29,000,000 x 850^4)
= (6000 x 27,000,000) / (48 x 29,000,000 x 522,006,250,000)
From there, it's simple math. My kid asked me how to do this. It's a
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