Question

which expression is not equivalent to 1/343? a. 7^4 x 7^-7 b. 7^7 x 7^-10 c. 7^2 x 7^-5. d. 7^-5 x 7^2

Answer 1

343=7*7*7=7³
1/343=1/7³=7^(-3)
the answer is equivalent to all of the above.
Please double check your question.  

Answer 2

1/343  = 1 / 7^3  or 7^-3

so a is equivalent,  so is b, c and d

they are all equivalent