Question

What is 1/3x -5/7 equal to

Answer 1

It can't be simplified because 1/3x and -5/7 aren't like terms and therefore you cannot do addition or subtraction with them.

If you meant x as in multiplied by then here is the solution:
1/3*-5/7
(1*-5)/(3*7)
-5/21

Hope this helps :)