I assume this is :
10*log10(1/[1+x^2])= 3 divide each side by 10
log10 ( 1 / [ 1 + x^2] ) = 3/10
In exponential form, this says that
10^(3/10) = 1 / [ 1 + x^2] and we can write
1 + x^2 = 1 / 10^(3/10) subtract 1 from both sides
x^2 = 1 / 10^(3/10) -1 take both pos/neg square roots and we have that
x = ±√ [1 / 10^(3/10) -1 ] ≈ ± .706 i [a non-real solution ]