.8^x = .0833
For exponential problems like this one, you can use logarithms to place the equation into a simple form more easily solved. Thus:.
log10(.8^x) = xlog10(.8) Any base number can be used, even 0.8, but tables are available for log bases 2, e, and 10
= (-.0969)x This can easily be obtained from a table of log values.
= log10(,0833) Given by the problem.
X = -1.079/(-.0969) = 11.1352