Answer
$d=-\frac{45}{7}$
Work Step by Step
This is a linear function. Solve for $d$. $$
\begin{aligned}
-1.5(d+5)-7 & =2 d+8 \\
-1.5(d+5) & =2 d+8+7 \\
-1.5(d+5) & =2 d+15 \\
-1.5 d-7.5 & =2 d+15 \\
-1.5 d-2 d & =15+7.5 \\
-3.5 d & =22.5 \\
d & =-\frac{22.5}{3.5}\\
& = -\frac{225}{35}\\
&= -\frac{45}{7}\\
&\approx -6.429.
\end{aligned}
$$ The solution is $$ d=-\frac{45}{7}.$$
Define the following version of the function and plot it. We see that the zero is where it should be. $$
f(d) = 2d+8+1.5(d+5)+7=3.5d+22.5.
$$
