### Linear Equations and Graphing

Sometimes we are asked to find the equation of a line given only the slope and a point on the line.

#### Your Turn:

An airplane begins descending at the rate of 2000 feet per minute. After 1 min it is 28,000 feet above the ground. Assume the plane continues at the same rate of descent. What is the equation of the line that represents the plane’s decent.

#### Solution:

We will start by using the slope-intercept form of line equation: $\color{blue}{y=mx+b}$

Where $x$ represents the time and $y$ represents the distance from the ground.

We are told that the plane is descending at the rate of 2000 feet per minute. That would represent the slope of the equation.

$\color{blue}{m=\frac{-2000 ft.}{1 min.}}$ |
$$ \begin{align*} y&=\color{blue}mx+b\\ y&=\color{blue}{-2000}x+b \end{align*} $$ |

Now we need to find a coordinate that will be on the line of the equation. Since the airplane was 28,000 feet above the ground after 1 minute, our coordinates are $(1,28000)$.

$$

\begin{align*}

\color{blue}y&=-2000\color{blue}x+b\\

\color{blue}{28,000}&={-2,000}*\color{blue}1+b

\end{align*}

$$

Now we can solve for b.

$$

\begin{align*}

28,000&={-2,000}*1+b\\

28,000\color{blue}{+2000}&={-2,000}\color{blue}{+2000}+b\\

\color{blue}{30,000}&=b

\end{align*}

$$

Substitute into the equation $y=mx+b$ to determine the equation of the line formed

$$

\color{blue}{y=-2,000x+30,000}

$$