Don't split the radical up until you subtract the exponents of the x's



Now split it up:



Since the power of x is a multiple of 7 we can take
it out of the radical on top, divide the exponent 21
by the index of the radical 7 and get x3 which
we write in front of the top radical:



Since 243 = 35, we replace 243 by that:



We have 35 on the bottom, and
we need 37 so that it will come out of the
radical, so we multiply by 



Multiply under the radicals:





Rewrite 8*9 as 72 and the denominator as just 3





Edwin