Simplify the log terms:
5−21log10(1+x)+2log10(1−x)=−21log10(1−x2)
Notice that log10(1−x2)=log10(1+x)+log10(1−x).
So,
5−21log10(1+x)+2log10(1−x)=−21log10(1+x)−21log10(1−x)
Cancel −21log10(1+x) from both sides:
5+2log10(1−x)=−21log10(1−x)
5=−25log10(1−x)
log10(1−x)=−2
1−x=10−2=0.01
x=0.99
Therefore, 100x=99.