24: The last operation is subtraction and then multiplication by 1/2. By linearity differentiate each part separately and pull 1/2 out of the first derivative.
The last operation in the first part is the logarithm composed with an inside function. Apply the chain rule.
The last operation in the second part is the arccos composed with an inside function. Apply the chain rule.

Alternative: A much smarter solution involves substitution. Take for y the expression that always comes with x, then start differentiating.

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