What is the math equation that satisfies conditions given below?

Question

Variable $z$ varies from $-50.0$ to $18.0$. Find the equation that gives $d$ such that when $z=-50$, $d = 0.1$; and when $z = 18.0$, $d = 1.0$. The values of $d$ vary from 0.1 to 1 uniformly depending on the value of $z$ with the boundary conditions given above.

Answer 1

As $d$ changes from $0.1$ to $1$ (a change of $1-0.1=0.9$), $z$ changes from $-50$ to $18$ (a change of $18--50=68$), so for every change of $1$ in $z$, $d$ changes by $\frac{0.9}{68}$. When $z=-50$, $d=0.1$. So, an equation relating $z$ and $d$ is $$d-0.1=\frac{0.9}{68}(z+50).$$