Well, what number can 7 and 5 multiply to? 35 right!? So you times 7 x 5 and 5 x 7 so both of the denominators will be 35. So, whatever you do to the bottom you have to do to the top. So you times 6 x 5 which makes the fraction -30/35 - 21/35. A negative plus a negative is still negative so... and you can definitely get the answer now! If you need any help just reply this and i'll help you. Oh and remember to keep the denominator 35 and to reduce your answer if you can! Hope this helps! (:
To solve this, you need to find the least common multiple of the denominators, which is 35 and divide by the denominators. What you get after dividing, multiply by the numerator. This will be;
-6/7 ? 3/5
-51/35 = -1 16/35
Get more examples on http://www.mathsisfun.com/fractions_subtraction.html.