The general X$-$momentum equation for a compressible fluid is: $\{$:$[$rho$($Du$)/($Dt$)=],[-($del p$)/($del x$)+($del$)/($del x$)($mu$($del u$)/($del x$))+($del$)/($del y$)($mu$($del u$)/($del y$))+($del$)/($del z$)($mu$($del u$)/($del z$))],[+[($del$)/($del x$)($mu$($del u$)/($del x$))+($del$)/($del y$)($mu$($del v$)/($del x$))+($del$)/($del z$)($mu$($del w$)/($del x$))]+($del$)/($del x$)($lambda divu$)]$:$\}$ Create a simplified X$-$momentum equation if the fluid is treated as incompressible and of constant viscosity. $[6$ marks$]$ a$.$ Swapping of mixed derivative. $(2$ marks$)$ b$.$ Apply the mass balance equation, div$($U$)=0.(2$ mark $)$ c$.$ The $3^("$rd $")$ and $4^("$th $")$ terms in the RHS of $($E$1)$ are cancelled. $(2$ marks$)$