#### Answer

The answer is $\frac{\text{dy}}{\text{dx}}=-\frac{9}{2}$.

#### Work Step by Step

First way:
Using Eq. (8) we have
$\frac{\text{dy}}{\text{dx}}=\frac{y'(t)}{x'(t)}=-\frac{9}{2}$.
Second way:
Since $x=2t+1$, so $t=(x-1)/2$. Substituting $t$ into $y=1-9t$ gives
$y=1-9 (x - 1)/2$,
$y=f(x)=-(9/2)x+11/2$.
Differentiating $f(x)$ we get
$\frac{\text{dy}}{\text{dx}}=-\frac{9}{2}$.