The derivative of the function in terms of t is,
2(4x)(dx / dt) + 2(9y)(dy / dt) = 0
Substituting the known values from the given above gives,
2(4)(2)(dx / dt) + 2(9)(2/3)(sqrt of 5)(1 /3) = 0
Solving the equation gives a value of dx/dt equal to -0.559.