#10^(x-6)=2^(x-6)5^(x-6)# then
#5^(x+3)/(5^(x-6)) > 2^(x-6)# but
#5^(x+3)/(5^(x-6)) =5^(x+3-(x-6)) = 5^9# then
#5^9 > 2^(x-6)# or
#5^9 2^6 > 2^x# now aplying #log# to both sides the inequality is preserved because #log# is monotonic strict increasing. So
#9log_2 5+6log_2 2>x# or finally
#x < 6+9log_2 5#