Question

[]-[]-[]=31using (2,4,6,8,10,12,14)

Answer 1

There are three operands,  and two  subtractions signs.  In the normal way of addition and subtraction with out use of some mathematical operators, it does not seem to be working out.

   ${}^{10}C_8$ - 12 - 2  = 31            combinations of selecting 8 items from 10, = 45

   64 - 28 - 10/2  =  31            used digits 6, 4, 2, 8, number 10 and a division operator.

   6² - 4 - Log10  = 31

   6²  - 2  -  Log_2 8  = 31              Logarithm of 8 to base 3.

   48  - 4²  - ${}^2C_2$        combinations of selecting 2 items from 2,  = 1.
   6*8  - 2⁴ -  ${}^2C_2$   

   8² / 2  - 2/4  - 2/4        = 31

     6²  -  12/ 6  - 12/4  =  31