He's right about your incomplete use of parentheses.  You should have

(1+sinx-cosx)/(1+sinx+cosx) = (sin2x(1-cosx))/(cosx(1-cos2x))

If you are writing an expression all on one line, then if a numerator or
denominator contains more than one letter or number, you must put a "(" 
to show where the numerator or denominator begins and a ")" to show where
it ends. I will now prove what you should have.

Since both sides are complicated, this is one in which we will have to work 
each side separately, simplifying each to the same expression.  
First we simplify the left side:

 = 



To make things easier use S = sin(x) and C = cos(x) on the left side:



Multiply by  which just equals 1

·









Use 1-S²=C² which is really 











Multiply by 







Now we simplify the right side:



We use sin(2x)=2sin(x)cos(x) and
cos(2x)=1-2sin²(x)











So we have reduced both sides to the same expression, so we
have proved the identity.

Edwin