the derivative, with respect to x, of is . You do not have the "3" that was in the original term in the second part.
I don't know where that " comes from. You had already done that in first "3y^3[/tex]". The derivative, with respect to x is, in fact -4 so the left side is
. The right is 20yy' as you say so you have
Subtract 10yy' and from both sides to get
Your only error appears to be that missing "3".