I need help simplifying log2 x = -1

I guess you mean solving:

log_2\ x = -1

?

Then, you need to know that if:

log_a\ b = c

then;

b = a^c

Can you try it now? Post your answer! :)

The long way round is:
log_a\ b = c

\frac{log\ b}{log\ a} = c

log\ b = c log\ a

log\ b = log\ a^c

b = a^c