solve difference equation for y$($k$)$ x$($k$+2)-1\mathrm{.}3$x$($k$+1)+0\mathrm{.}4($k$)=$u$($k$)$ where x$(0)=$x$(1)=0$ x$($k$)=0$; k$<0.$ u$($k$)=1$; k$=0,1,2....$ u$($k$)=0$; k$<0$