The system of equations $$ x+2 y+3 z=6, x+3 y+5 z=9 \text {, } $$ $2 x+5 y+a z=12$ has no solution when $a=$

If $m, n$ are respectively the least positive and greatest negative integer value of $k$ such that $\left(\frac{1-i}{1+i}\right)^k=-i$, then $m-n=$

If a complex number $z$ is such that $\frac{z-2 i}{z-2}$ is purely imaginary number and the locus of $z$ is a closed curve, then the area of the region bounded by that closed curve and lying in the first quadrant is $\frac{z-2 i}{z-2}$

Real part of $\frac{(\cos a+i \sin a)^6}{(\sin b+i \cos b)^8}$ is