how do I raise a number to a power like -7?

Have a look at this thread.

http://web2.0calc.com/questions/indices-especially-negative-indices

Note that, for example,  2^-7   =   1 / 2⁷  = 1 / 128

So we can always write ("1") over (our number raised to the same positive exponent.)

Of course, a calculator can be used, too !!!!