Note $t(1)$ is given.

$t(1 + 1) = t(1) − 5$
or $t(2) = 12 − 5$

$t(2 + 1) = t(2) − 5$
or $t(3) = 7 − 5$

Note first term $a_{1}$ is $32$.

To find $a_{2}$, substitute $32$ into the equation.

$a_2=\frac{1}{2}(32)=16$