1. The problem statement, all variables and given/known data Suppose z=ψ(2x-3y), Show that the second partial derivative of z with respect to x, is equal to the second partial derivative with respect to y multiplied by a scalar k. 2. Relevant equations 3. The attempt at a solution I thought this was too simple to be correct, and apparently it was. I just use the chain rule and found the first partial derivative with respect to x as 2ψ and with respect to y as -3ψ, hence both the second partial derivatives will be 0, and hence the scalar k is just 0. But apparently this is incorrect, apparently the scalar k is 4/9. I'm not exactly sure why, but our maths teacher told us to redefine z in terms of u, and then make u depend on x and y. Any help would be appreciated. Cheers.