General term for the given binomial is
Tr+1=6Cr(x1+log10x1)6−r(x121)r
Given T4=200, putting r=3
⇒T4=6C3(x1+log10x1)3(x121)3=200
⇒(x1+log10x1)23⋅(x41)=10
⇒x2(1+log10x)3+41=10
⇒(2(1+log10x)3+41)log10x=1
⇒x=10,10−4
⇒x=10 ( as x>1)