Verify that 4x^2 + 7y^2 = C satisfies the differential equation 4x + 7yy' = 0

Initial condition y = 4 when x = 5, what must C be?

C = ?

Could you please show step by step work?

This is what I understand so far. You're suppose to move the equation so that y' = -4x/(7y)

and then use implicit differentiation? Then use the initial conditions to solve for C?

I've attached my work with what I have so far but I think I messed up somewhere at the really beginning

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