Impulse response of a time scaling system
The input-output relation of such a system can be written as

$$y(t)=\int_{-\infty}^{\infty}\delta(t/2-\tau)x(\tau)d\tau\tag{1}$$

Note that the system is linear but time-varying, and such systems can generally be described by a two-dimensional impulse response $h(t,\tau)$:

$$y(t)=\int_{-\infty}^{\infty}h(t,\tau)x(\tau)d\tau\tag{2}$$

In the given example we simply have

$$h(t,\tau)=\delta(t/2-\tau)\tag{3}$$

For linear time-invariant (LTI) systems, $h(t,\tau)$ only depends on the difference $t-\tau$.