Answer
$y=e^{x/2}\\ z=e^x\\z=y^2$
Work Step by Step
Given: $x=2t; y=e^t; z=e^{2t}$
Consider $y=e^{(t)}=e^{(2t/2)}=e^{\frac{x}{2}}$
Now, next we have $z=e^{2t}\implies z=e^x$
Also, $z=e^{2t} $
This yields: $ z=(e^{(t)})^2=y^2$
Therefore, $y=e^{x/2}\\ z=e^x\\z=y^2$