So the error they made is that they didn't multiply 5 by 2 when expanding the brackets, they just multiplied 5 by x and then added the 2. You can phrase this how you want. The expression written correctly would be 5x +/- 10 (it's +/- because I can't see which operation is in between x and 2, whichever it is, put that in front of 10)
Hope this helps!
