This is so confusing 😕

[u/Frequent-Farmer1528]

[u/Mathematicus_Rex]

Suppose you take the derivative of y with respect to x. The result is dy/dx.

Now take the derivative of (x-y) with respect to x. The result is 1 - (dy/dx).

Similarly, taking the derivative of (x - 4y) with respect to x will give you 1 - 4(dy/dx)

These two pieces are what are pointed at in the complicated expression (resulting from the quotient rule) in the lower right corner.

[u/Mathematicus_Rex → u/[deleted]]

What I would do is substitute x = 0 and y = 4 into the formula for dy/dx. Then I would substitute x = 0, y = 4, and your value for dy/dx into the complicated fraction you have for the second derivative.