can u help me for calculating its z transform
You want the one sided z-transform of $\displaystyle x_n=d^n,\ n=0, 1, ...\ $
The z-transform of this is:
$\displaystyle \displaystyle \left(\mathcal{Z}(x_n) \right)(z)=\sum_{n=0}^{\infty} x_n z^{-n}$ $\displaystyle \displaystyle
=\sum_{n=0}^{\infty} \left(\frac{d}{z}\right)^n$
You should recognise the right hand side as a geometic series which converges when $\displaystyle d<z$ and you should know how to sum that.
CB