No, your denominator 10x -4 is dividing into both terms of the numerator. There for you may seperate the expression to 5x/10x-4 + 2/10x-4. Let me try and expain in with some different numbers lets say we have the fractions 1/4 + 2/4 can be combined to 3/4. Now instead of using the number 4 lets use the term (1+3) as the denonminator and instead of using the numerator 3 lets use 2 + 1.So now we can have 1/(1+3) + 2/(1+3) = 3/(1+3). Or (2+1)/(1+3). However if we seperate the denominator and the numerator of the answer we would get 2/1 + 1/3 which would equal 2 and 1/3 which is not equal to 3/4. Does this make sense?