##### Definition

An equation that describes a straight line on 2D surface with x and y co-ordinates is the equation of straight line. The equation of straight line can be calculated using various equations :

- Slope Intercept Form
- Point Slope Form
- Two Point Slope Form

As the name indicates, an equation which can be represented with the slope and slope intercept is calculated using the slope intercept form.

#### Formula :

Slope Intercept Form y = mx + c
**Where,**
m = Slope
x = X Axis Points
c = Y-intercept
###### Example :

Find the equation if slope is 4 and y-intercept is 7
Given,
m = 4 and c = 7
**Solution,**
y = mx + c
y = 4x + 7
4x-y+7 = 0 is the equation of straight line.

Equation of straight line formed by one co-ordinate point and slope is called as point slope form. It is expressed as

#### Formula :

(y - y1) = m(x- x1)
**Where,**
m = Slope
x_{1} = X Axis Points
y_{1} = Y Axis Points
x, y = x and y Axis
###### Example :

Let slope be 7 and Co-ordinate point be (3,4), then
Given, m = 7, x1 = 3 and y1 = 4
**Solution,**
__Step 1__:
Apply the values in (y - y1) = m(x- x1)
(y-4) = 7 ( x-3)
__Step 2__:
Simplifying,
(y-4) = 7x - 21
y-7x = -21 + 4
y-7x = -17
7x-y-17 = 0 is the equation.

When two co-ordinate points are given, we can find straight line equation using two point slope form.

#### Formula :

(Y - Y_{1}) / (Y_{2} - Y_{1}) = (X - X_{1}) / (X_{2} - X_{1})
**Where,**
m = Slope
x_{1} = X Axis Points
y_{1} = Y Axis Points
x, y = x and y Axis
###### Example

Let consider the axis point (x1,y1) and (x2,y2) as (5,6) and (7,8), then
Given, x1 = 5, y1 = 6, x2 = 7 and y2 = 8
**Solution,**
(Y - 6) / (8 - 6) = (X - 5) / (7 - 5)
(Y - 6) / 2 = (X - 5) / 2
(Y - 6) = (X - 5)
x - y - 11 = 0

Slope describes the steepness and direction of a straight line. We can find the slope when two co-ordinate points are given.

#### Formula :

m = (Y_{1} - Y_{2}) / (X_{1} - X_{2})
**Where,**
m = Slope
X_{1} , X_{2} = X Axis Points
Y_{1}, Y_{2} = Y Axis Points
###### Example :

Let consider the points (2,3) and (7,6).
Given, x1 = 2, y1 = 3, x2 = 7 and y2 = 6
**Solution,**
m = 3 - 6 / 2 - 7
m = -3 / -5
m = 3/5 = 0.6