How can I solve for x?

The general form of your equation is


A = B(1-C)^x


where A. B and C are known constants. This can be rearranged as follows:


\frac A B = (1-c)^x


I'll stop here, and assuming that you are studying logarithms think about what happens if you take the log of both sides.
Post back with your attempt at manipulatuing this and we'll check it for you.

I know how to use logarithm.
I use log 0.107 on both sides:
log5/28= xlog 0.107
log0.107 log 0.107

and I got X= 0.772

Good, although I find that log(5/28)/log(0.107) = 0.771.