Answer
$(ABx)^{T}$=$x^{T}B^{T}A^{T}$
Work Step by Step
Apply Th.3.d, $(AB)^{T}=B^{T}A^{T}$
replacing the matrix A with AB, and the matrix B with the matrix $x$
$(ABx)^{T}=[(AB)x]^{T}$
$=x^{T}(AB)^{T}$
... apply Th3.d again
$=x^{T}B^{T}A^{T}$
$(ABx)^{T}$=$x^{T}B^{T}A^{T}$
