Question

The percent of women in the U.Ss who were in the civilian labor force increased rapidly for several decades and then stabilized. If x represents the number of years since 1950, the function: f(x)=67.21/1+1.081e^-x/24.71
a.) What percent of U.S. women were in the civilian labor force in 2008?
b.) In what year were 55% of U.S. women in the civilian labor force?

How do I isolate X in b.)?