Question

how do i write an expression using a negative exponent

Answer 1

With a negative exponent, you need to create a fraction. Put the whole expression over one, like this - 
(expression)/1 or 
(expression)
__________
         1.
From there, you need to put the base and negative exponent on the bottom of the fraction with the 1, but turn the exponent into a positive. Hope this helps.