when you multiply exponents with the same base you just add the exponents. ex: 2^2 x 2^4=2^6 in this equation you have the base 6 so now you just have to add ur exponents. you have -5/4 and -8/7 but in order to add them you have to make them have the same denominator. you multiply -5/4 by 7/7 and -8/7 by 4/4 (this way they both have the denominador 28) and get -35/28 and -32/28. Now you add them together and you get -67/28 making ur answer 6^-67/28
