The value  $k$ for which  $4x^2+8xy+k{y}^2=9$ is the equation of a pair of straight lines is ____

The equation  $4x^2+8xy+k{y}^2-9=0$ represents a pair of straight lines if $∆=0$

  $\Rightarrow \left(4\right)\left(k\right)(-9)-(-9){\left(4\right)}^2=0$

$\Rightarrow$  $k=4$