$axy\left(ax-yx+ay\right) \mathrm{at} a=1$ is

$axy(ax – yx + ay)$

$=axy\times \left(ax\right)–axy\left(yx\right)+axy\left(ay\right)$

$=a^2{x}^{2}y-a{x}^2{y}^2+a^{2}x{y}^2$

On putting $a=1$ in the above expression, we obtain

$a^2{x}^{2}y-a{x}^2{y}^2+a^{2}x{y}^2$

$={\left(1\right)}^2{x}^{2}y-\left(1\right)x^2{y}^2+{\left(1\right)}^{2}x{y}^2$

$=x^{2}y-x^2{y}^2+x{y}^2$

Therefore, the expression $axy(ax – yx + ay)$ at $a=1$ is ${\mathit{x}}^{\mathbf{2}}\mathit{y}\mathbf{-}{\mathit{x}}^{\mathbf{2}}{\mathit{y}}^{\mathbf{2}}\mathbf{+}\mathit{x}{\mathit{y}}^{\mathbf{2}}.$