Create a linear relationship (equation) that you would use or rely on in your field...

Step 1
I have a monthly bus pass to travel around the city. My buss pass changes a fixed a mount of $200 and a variable amount of $1.5 per mile.
Monthly cost of my travel can be obtained by wing linear equation.
If in a month i travel x miles then the total monthly cost is
$$\begin{gathered} \text{Fixed change}=\$200 \\ \text{Variable charge}=\$1.5\times (x)=\$1.5x \end{gathered}$$
Step 2
Therefore total monthly cost c(x) for x miles in a month
$c(x)=1.5x+200---(1)$
If in a certain month i dont travel at all then, $x=0$
Therefore, still i have to pay $200, which is interval condition.
From equation (1), we can find slope is 1.5. Hence rate of change is $1.5 per mile.