what is wrong with the way this derivative written?

Ronen Toren wrote: Why it not solved?

It's difficult to tell from a screen shot - it's much better all round if you could post the worksheet.

However, I suspect (as d and dx both look italic) that you have typed d/dx manually rather than using the d/dx operator (in the Math ribbon under the Operators drop-down).

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Note that the d/dx in my case is in an upright, not italic, font.

Stuart