Answer
$(4t-3)(2t-7)$
Work Step by Step
Grouping the first and second terms and the third and fourth terms, the given expression is equivalent to
\begin{array}{l}\require{cancel}
8t^2-6t-28t+21
\\\\=
(8t^2-6t)-(28t-21)
.\end{array}
Factoring the $GCF$ in each group results to
\begin{array}{l}\require{cancel}
2t(4t-3)-7(4t-3)
.\end{array}
Factoring the $GCF=
(4t-3)
$ of the entire expression above results to
\begin{array}{l}\require{cancel}
(4t-3)(2t-7)
.\end{array}