Calculate the Discrete$-$Time Fourier Transform $($DTFT$)$ of the following discrete$-$time signals:

$x_1[n]=3*6^n*u[-n-2*6]$

$x_2[n]=cos(\frac{2n}{6}+3)+sin(\frac{n}{6}+2)*[u[n+6]-u[n-$

$6]]$

Next, calculate the discrete$-$time signals corresponding to the following DTFT:

$x_3()=co{s}^2(+\frac{6}{10})$

$x_4()={\displaystyle \underset{k=-}{\overset{}{}}}[(-1{)}^k*(-\frac{k}{2})]$

Please note that $u[n]$ represents the unit step function, and $()$

is the Kronecker delta function.