$x-7$  

 $x^2-7$  

 $x+7$  

 $x^2+3x-54$  

 $x+5+\frac{2}{2x-4}$  

 $x-8$  

 $x-5+\frac{3}{2x-4}$  

 $x+5$   

This is correct!

This is incorrect, and error occurs on the 2nd line!

This is incorrect, and the error occurs on the 4th line!

This is incorrect, and the error occurs on the 3rd line!