Actually neither of those is very clear. Since this board does not implement LaTeX, use parentheses.
I think you mean 5/x+ (6/x^2)/(25/x^2)- 35/x.
The "complex fraction" (6/x^2)/(25/x^2), which has the fraction 6/x^2 in the numerator and 25/x^2 in the denominator, is interpreted as "divide 6/x^2 by 25/x^2". And you divide fractions by "invert the denominator and multiply": (6/x^2)(x^2/25) The two x^2 terms cancel leaving 6/25.
Now, can you find 5/x+ 6/25- 35/x? The "least common denominator" is 25x.