Hi, thank you for the answer, but do you have the steps for the question ? :)

Let's look at del z/ del x as an example. Since you're differentiating wrt x, you will treat y like as if it's a constant (i.e. real number). Differentiating ax^2 will therefore be 2ax. That should be straightforward. To differentiate 2bxy, you need to consider the expression as (2by)x instead, where y is grouped together with the constants. So upon differentiation of (2by)x, you obtain 2by. So the key takeaway is this: When performing partial differentiation wrt to a variable, treat all other variables as constants and differentiate as per normal.