Answer:
 m^2
 ----------- .
 3 n^4
Explanation:
The easiest approach here to obtain only positive exponents is to invert the entire quantity inside parentheses:
From the given expression we get the equivalent expression
 1
 -------------------- .
 3 m^(-2) n^4
 1
Given that we want only positive exponents, rewrite ------------ as m^2:
 m^(-2) 
 m^2
 ----------- .
 3 n^4
This is the final answer.