i am just writing a soln.. but don't know weather its right or wrong..
clearly, 21+x + 21-x + 31+x + 31-x are all positive and real.. for real x .. so why not A.M-G.M
so, we get, 51+x + 51-x > (36)1/4
or, 5x + 5-x > 61/2/5
or, 5x > 61/2/5 - 5-x
=> x > log5(61/2/5 - 5-x)
comparison can be made only if the terms are real...
and for R.H.S to be real...
the definiton of log should be used...
i.e, 5-x < 61/2/5
again, taking logs...
i know its too much but there's no harm if are confirmed that the terms are positive and also the bases i.e. greater than 0 less than 1 or greater than 1...
=> -x < log561/2 - 1
=> x > 1 - log561/2
this is the x for which we got x to be defined for...
but i don't think that this the answer as well...